Amendments and Nonconformance Penalties for Model Year 2027 and Later Heavy-Duty Highway Engines and Amendments to Inducement Provisions for SCR-Equipped Diesel Engines

Federal RegisterJul 14, 2026

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 86, 1036, 1037, 1039, 1065, and 1071

[EPA-HQ-OAR-2026-0728; FRL 12756-01-OAR]

RIN 2060-AW83

Amendments and Nonconformance Penalties for Model Year 2027 and Later Heavy-Duty Highway Engines and Amendments to Inducement Provisions for SCR-Equipped Diesel Engines

AGENCY:

Environmental Protection Agency (EPA).

ACTION:

Proposed rule.

SUMMARY:

The U.S. Environmental Protection Agency (EPA) is proposing regulatory amendments to certain compliance provisions and test procedures related to model year (MY) 2027 and later heavy-duty highway engines. These amendments would include changes to the regulatory useful life periods and the emission-related warranty periods. The EPA also proposes to add clarity to certain regulatory compliance provisions and correct errors in the regulations to support the MYs 2027 and later program for heavy-duty highway engines and vehicles. This includes certain amendments related to provisions adopted in January 2023 as well as other provisions adopted in earlier rules. The EPA also proposes to make nonconformance penalties (NCPs) available to manufacturers of medium heavy-duty engines (Medium HDE) and heavy heavy-duty engines (Heavy HDE) beginning in MY 2027. In addition, the EPA proposes to amend the requirements for selective catalytic reduction (SCR) system inducement provisions for newly manufactured diesel-fueled highway engines and vehicles (

i.e.,

light- and medium-duty vehicles and heavy-duty engines) and nonroad engines and equipment. The EPA is also considering new inducement guidance for in-use highway and nonroad diesel engines, vehicles, and equipment.

DATES:

Comments. Comments must be received on or before August 29, 2026. Comments on the information collection provisions submitted to the Office of Management and Budget (OMB) under the Paperwork Reduction Act (PRA) are best assured of consideration by OMB if OMB receives a copy of your comments on or before August 13, 2026.

Public hearing: The EPA will hold virtual public hearings on July 29, 2026, starting at 9:00 a.m. Eastern Time and on July 30, 2026, starting at 10:00 a.m. Eastern Time. If there is sufficient interest, an additional day of hearings will be held on the subsequent day. Please refer to the

SUPPLEMENTARY INFORMATION

section for additional information on the public hearing. Information on the status of the hearing and how to register can be found at

https://www.epa.gov/regulations-emissions-vehicles-and-engines/proposed-rule-amendments-and-nonconformance-penalties.

ADDRESSES:

You may send comments, identified by Docket ID No. EPA-HQ-OAR-2026-0728, by any of the following methods:

•

Federal eRulemaking Portal:

www.regulations.gov

(our preferred method). Follow the online instructions for submitting comments.

•

Email:

a-and-r-Docket@epa.gov.

Include Docket ID No. EPA-HQ-OAR-2026-0728 in the subject line of the message.

•

Mail:

U.S. Environmental Protection Agency, EPA Docket Center, OAR Docket EPA-HQ-OAR-2026-0728, Mail Code 28221T, 1200 Pennsylvania Avenue NW, Washington, DC 20460.

•

Hand Delivery or Courier:

EPA Docket Center, WJC West Building, Room 3334, 1301 Constitution Avenue NW, Washington, DC 20004. The Docket Center's hours of operation are 8:30 a.m.-4:30 p.m., Monday-Friday (except Federal holidays).

Instructions:

All submissions received must include the Docket ID No. for this rulemaking. Comments received may be posted without change to

www.regulations.gov,

including personal information provided. For detailed instructions on sending comments and additional information on the rulemaking process, see the “Public Participation” heading of the

SUPPLEMENTARY INFORMATION

section of this document.

FOR FURTHER INFORMATION CONTACT:

For information about this proposed rule, contact James Sanchez, Transportation Sector Impacts & Standards Division, Office of Transportation and Air Quality, Environmental Protection Agency, 2000 Traverwood Drive, Ann Arbor, MI 48105; telephone number: (734) 214-4439; email address:

Sanchez.James@epa.gov.

SUPPLEMENTARY INFORMATION:

Public Participation

A. Written Comments

Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2026-0728, at

www.regulations.gov

(our preferred method), or the other methods identified in the

ADDRESSES

section. Once submitted, comments cannot be edited or removed from the docket. The EPA may publish any comment received to the Agency's public docket. Do not submit to the EPA's docket at

www.regulations.gov

any information you consider to be Confidential Business Information (CBI), Proprietary Business Information (PBI), or other information the disclosure of which is restricted by statute. If you choose to submit CBI or PBI as a comment to the EPA's docket, please send those materials to the person listed in the

FOR FURTHER INFORMATION CONTACT

section. Multimedia submissions (audio, video, etc.) must be accompanied by a written comment. The written comment is considered the official comment and should include discussion of all points you wish to make. The EPA will generally not consider comments or comment contents located outside of the primary submission (

i.e.,

on the web, cloud, or other file sharing system). Please visit

www.epa.gov/dockets/commenting-epa-dockets

for additional submission methods; the full EPA public comment policy; information about CBI, PBI, or multimedia submissions; and general guidance on making effective comments.

B. Participation in Virtual Hearing

The EPA will announce detailed information about the hearing as described in

ADDRESSES

. The EPA will begin pre-registering speakers for the hearing upon publication of this document in the

Federal Register

. To register to speak at the virtual hearing, please send an email to

EPA-HD-MobileSource-Hearings@epa.gov.

If you need additional help, contact the person listed in the

FOR FURTHER INFORMATION CONTACT

section.

Registration will be open through the last day of the hearing; however, the EPA asks that you pre-register by July 22, 2026 if you intend to testify or are requesting special accommodations such as the services of an interpreter or audio description, please pre-register for the hearing and describe your needs by July 22, 2026. The EPA may not be able to arrange accommodations without advance notice.

The EPA will provide a general agenda for the hearing listing the pre-registered speakers in approximate order at

https://www.epa.gov/regulations-emissions-vehicles-and-engines/proposed-rule-amendments-and-nonconformance-penalties

before the first day of the hearing. The EPA will make every effort to follow the schedule as closely as possible on the day of the hearing; however, please plan

for the hearings to run either ahead of schedule or behind schedule. Registration will be open through the last day of the hearing and the EPA will make every effort to accommodate all speakers who join the hearing, although preferences on speaking times may not be able to be fulfilled, and special accommodations may not be available.

Each commenter will have up to three minutes to provide oral testimony, though the final time limit may change and will be provided to registered speakers before the first day of the hearing. The EPA may ask clarifying questions during the oral presentations but will not respond to the presentations at that time. Written statements and supporting information submitted during the comment period will be considered with the same weight as oral comments and supporting information presented at the public hearing.

Please note that any updates made to any aspect of the hearing are posted online at

https://www.epa.gov/regulations-emissions-vehicles-and-engines/proposed-rule-amendments-and-nonconformance-penalties.

While the EPA expects the hearing to go forward as set forth above, please monitor the Agency's website to determine if there are any updates. The EPA does not intend to publish a document in the

Federal Register

announcing updates.

C. Public Access to Voluntary Consensus Standards

Copies of ASTM standards identified in section III.E of this preamble may be obtained from ASTM International, 100 Barr Harbor Dr., P.O. Box C700, West Conshohocken, PA 19428-2959, by calling (877) 909-ASTM, or at

www.astm.org.

Copies of these standards have also been placed in the rulemaking docket for this action. For the latest status information on EPA Docket Center Reading Room services and docket access, visit

www.epa.gov/dockets/epa-docket-center-reading-room

or call (202) 566-1744. If you have a disability and the format of any of these materials interferes with your ability to access the information, please contact the EPA's Rehabilitation Act Section 508 (29 U.S.C. 794d) Program at

www.epa.gov/accessibility/forms/contact-us-about-section-508-accessibility

or via email at

section508@epa.gov.

To enable the EPA to respond in a manner most helpful to you, please indicate the nature of the accessibility issue, the web address of the requested material, your preferred format in which you want to receive the material (electronic format (ASCII, etc.), standard print, large print, etc.), and your contact information.

D. Docket

All documents in the docket are listed on the

www.regulations.gov

website. A plain language summary of the rule is also available on the

www.regulations.gov

website. Although listed in the index, some information is not publicly available,

e.g.,

CBI or other information the disclosure of which is restricted by statute. Certain other material, such as copyrighted material, is not placed on the internet and will be publicly available only in hard copy form through the EPA Docket Center at the location listed in the

ADDRESSES

section of this document.

General Information

Does this action apply to me?

This action relates to companies that manufacture, sell, or import into the United States motor vehicles, motor vehicle engines, and nonroad engines and equipment. Potentially affected categories and entities include the following:

NAICS codes

Sector title

333618

Other Engine Equipment Manufacturing.

336110

Automobile and Light Duty Motor Vehicle Manufacturing.

336120

Heavy Duty Truck Manufacturing.

336310

Motor Vehicle Gasoline Engine and Engine Parts Manufacturing.

336390

Other Motor Vehicle Parts Manufacturing.

336999

All Other Transportation Equipment Manufacturing.

333111

Farm Machinery and Equipment Manufacturing.

333120

Construction Equipment Manufacturing.

NAICS Association. NAICS & SIC Identification Tools.

www.naics.com/search.

This table is not intended to be exhaustive but rather provides a guide for readers regarding entities potentially affected by this action. This table lists the types of entities that the EPA is aware could potentially be affected by this action. Other types of entities not listed in the table could also be affected. To determine whether your entity is regulated by this action, you should carefully examine the applicability criteria found in the regulation. If you have questions regarding the applicability of this action to a particular entity, consult the person listed in the

FOR FURTHER INFORMATION CONTACT

section.

What action is the Agency taking?

The EPA is proposing regulatory amendments to compliance provisions and test procedures related to the Agency's 2023 rule setting criteria pollutant emission standards for MYs 2027 and later heavy-duty highway engines. These amendments are intended to add clarity to the regulatory compliance provisions and correct errors in the regulations.

What is the Agency's authority for taking this action?

Statutory authority for this proposed action comes from the Clean Air Act (CAA) (42 U.S.C. 7401-7671q).

List of Acronyms

Acronym

Description

AAF

Annual Adjustment Factor

ABT

Averaging, Banking, and Trading

AECD

Auxiliary Emissions Control Device

ANSI

American National Standards Institute

ASTM

ASTM International

ATA

American Trucking Associations

CAA

Clean Air Act

CARB

California Air Resources Board

CCR

California Code of Regulations

CFR

Code of Federal Regulations

CNG

Compressed Natural Gas

Charge Depleting

Carbon Monoxide

COC

Cost of Compliance

Charge Sustaining

DEF

Diesel Exhaust Fluid

DPF

Diesel Particulate Filter

DRIA

Draft Regulatory Impact Analysis

DTSD

Draft Technical Support Document

EMA

Truck & Engine Manufacturers Association

EGR

Exhaust Gas Recirculation

EPA

Environmental Protection Agency

FEL

Family Emission Limit

Federal Register

FTP

Federal Test Procedure

GVWR

Gross Vehicle Weight Rating

Hydrocarbons

Heavy-Duty

HDE

Heavy-Duty Engine

ICP-MS

Inductively Coupled Plasma Mass Spectrometer

ISO

International Organization for Standardization

LLC

Low Load Cycle

Marginal Cost

MIL

Malfunction Indicator Light

MOVES

MOtor Vehicle Emission Simulator

Model Year

NCP

Nonconformance Penalty

Natural Gas

NMHC

Nonmethane Hydrocarbon

Oxides of Nitrogen

NPRM

Notice of Proposed Rulemaking

NTTAA

National Technology Transfer and Advancement Act

OBD

Onboard Diagnostics

OEM

Original Equipment Manufacturer

OMB

Office of Management and Budget

PCA

Production Compliance Audit

Particulate Matter

Penalty Rate

PRA

Paperwork Reduction Act

RESS

Rechargeable Energy Storage System

RFA

Regulatory Flexibility Act

RIA

Regulatory Impact Analysis

RPE

Retail Price Equivalent

SAE

SAE International

SCR

Selective Catalytic Reduction

SET

Supplemental Emission Test

Sulfur Dioxide

SVOC

Semi-Volatile Organic Compounds

THC

Total Hydrocarbon Emissions

TSD

Technical Support Document

Upper Limit

ULSD

Ultra Low Sulfur Diesel

UMRA

Unfunded Mandates Reform Act

UQS

Urea Quality Sensor

VOC

Volatile Organic Compounds

Table of Contents

I. Executive Summary

A. Introduction

B. Need for Regulatory Action

C. Summary of the Major Provisions in This Proposal

D. Summary of the Impacts of This Proposal

II. Introduction

A. Background

B. Response to the 2023 Final Rule

C. Summary of the Proposal

D. Legal Authority

E. Reliance Interests

III. Proposed Compliance Provisions and Test Procedures

A. Emission-Related Warranty

B. Regulatory Useful Life Periods

C. Production Volume Allowance

D. Credit Flexibilities

E. Other Regulatory Amendments

IV. Nonconformance Penalties for Diesel-Fueled Medium HDE and Heavy HDE

A. NCP Eligibility

B. Nonconformance Penalty Rates

C. Consideration of Other Methodologies and Factors for Estimating MY 2027 NCPs

D. Migrating Regulation to 40 CFR Part 1071 and Proposed Revisions to the Regulation

V. Improvements for SCR Reliability and Changes to DEF Inducements for Highway Engines and Vehicles and for Nonroad Diesel Engines

A. Background

B. Recent Developments

C. The EPA's Proposed Inducement Revisions

D. Proposed Revisions To Improve SCR Reliability

E. Applying Proposed SCR Inducement Changes to Light-Duty Vehicles, Medium-Duty Vehicles, and Nonroad Engines

F. Potential for Additional EPA Inducement Guidance for In-Use Engines and Vehicles

VI. Program Costs

VII. Estimated Emissions Changes From the Proposed Program Amendments

A. Emission Inventory Methodology

B. Emission Inventory Impacts

VIII. Air Quality Impacts of the Proposed Rule

IX. Projected Changes in Human Health and Welfare From the Proposed Rule

X. Economic Impact Analysis

XI. Summary of Requests for Comment in This Proposed Rule

XII. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

B. Executive Order 14192: Unleashing Prosperity Through Deregulation

C. Paperwork Reduction Act (PRA)

D. Regulatory Flexibility Act (RFA)

E. Unfunded Mandates Reform Act (UMRA)

F. Executive Order 13132: Federalism

G. Executive Order 13175: Consultation and Coordination With Indian Tribal Governments

H. Executive Order 13045: Protection of Children From Environmental Health and Safety Risks

I. Executive Order 13211: Actions Concerning Regulations That Significantly Affect Energy Supply, Distribution, or Use

J. National Technology Transfer and Advancement Act (NTTAA) and 1 CFR Part 51

XIII. Statutory Provisions and Legal Authority

List of Subjects

I. Executive Summary

A. Introduction

In this action, the EPA proposes regulatory amendments to certain compliance provisions and test procedures related to MYs 2027 and later heavy-duty highway engines. These amendments would include changes to the regulatory useful life periods and the emission-related warranty periods. The EPA also proposes to add clarity to certain regulatory compliance provisions and correct errors in the regulations to support the MYs 2027 and later program for heavy-duty highway engines and vehicles. This includes certain amendments related to provisions adopted in January 2023 and other provisions adopted in earlier rules. The EPA also proposes to make NCPs available to manufacturers of Medium HDE and Heavy HDE beginning in MY 2027. In addition, the EPA proposes to amend the requirements for SCR system inducement provisions for newly manufactured diesel-fueled highway engines and vehicles and nonroad engines and equipment.

The EPA is also considering new inducement guidance for in-use highway and nonroad diesel engines, vehicles, and equipment.

Throughout this preamble, the term “inducements” refers to SCR-related strategies to induce operators to maintain appropriate levels of high-quality DEF and not tamper with SCR systems. Engine derating and vehicle speed restrictions have historically been applied as the primary inducement for SCR systems. Engine derating is a design strategy that reduces engine power.

B. Need for Regulatory Action

In January 2023, the EPA issued a final rule titled “Control of Air Pollution from New Motor Vehicles: Heavy-Duty Engine and Vehicle Standards” (the “2023 Final Rule”).

The 2023 Final Rule included new, more stringent emission standards for oxides of nitrogen (NO

), particulate matter (PM), hydrocarbons (HC), and carbon monoxide (CO) for MY 2027s and later heavy-duty engines.

The 2023 Final Rule also changed key provisions of the existing heavy-duty engine emissions control program, including test procedures, regulatory useful life, emission-related warranty, and other requirements. Additionally, the 2023 Final Rule included changes to maintenance and serviceability, including newly codified SCR inducement provisions.

88 FR 4296 (Jan. 24, 2023).

The EPA refers to NO

, PM, HC, CO, volatile organic compounds (VOC), and sulfur dioxide more generally as “criteria pollutants” throughout this preamble.

Immediately upon taking office, President Trump established new Executive Branch priorities for energy, transportation, and consumer choice and committed to ensuring regulations remain within constitutional and statutory bounds. On March 12, 2025, the EPA Administrator announced that the Agency would reevaluate the 2023 Final Rule as part of the deregulatory actions the Agency would undertake.

Following this announcement, the EPA heard concerns from truck and engine manufacturers, the component supply industry, as well as truck users, trucking fleets, and truck dealerships, about the expected cost increases associated with the 2023 Final Rule's changes to the emission-related warranty and regulatory useful life periods (the Agency also received comments from a number of these stakeholders raising concerns with the potential high costs

during the development of the 2023 Final Rule). In addition, several engine manufacturers have expressed concerns with the increased useful life which is set to begin in MY 2027 and the increased risk of non-compliance for in-use engines over their extended regulatory useful life. The EPA has also identified several areas where amending specific test procedures and other compliance provisions could support implementation of the MY 2027 program.

U.S. Environmental Protection Agency. (2025). “EPA Launches Biggest Deregulatory Action in U.S. History”.

https://www.epa.gov/newsreleases/epa-launches-biggest-deregulatory-action-us-history.

On August 12, 2025, the Administrator also stated the EPA's intention to further improve operator experience with diesel engines using SCR systems, including assessing whether engine performance derates may no longer be necessary to ensure the proper use of diesel exhaust fluid (DEF).

After this announcement, individual operators and States, such as Iowa, Alaska, and Nebraska, expressed concerns with inducements, and a bill was brought to Congress regarding relief for inducements in prolonged cold weather.

In response to feedback and concerns that the EPA continues to receive regarding the impact of inducements, the Agency is revisiting the inducement regulations finalized in the 2023 Final Rule for new heavy-duty highway engines. This includes revisiting how restrictive inducements should be to provide a reasonable basis for demonstrating that operators would continue to add quality DEF in tanks and not tamper with SCR systems. The EPA is also considering what changes could apply to in-use highway engines and vehicles and nonroad engines currently covered under existing EPA guidance.

U.S. Environmental Protection Agency. (2025). “At Iowa State Fair, EPA Administrator Zeldin Announces Diesel Exhaust Fluid (DEF) Fix.”

https://www.epa.gov/newsreleases/iowa-state-fair-epa-administrator-zeldin-announces-diesel-exhaust-fluid-def-fix.

See,

e.g.,

Office of the Nebraska Attorney General (2026). “Nebraska Attorney General's Office Issues Safety Alert Regarding Diesel Exhaust Fluid (DEF) System Failures Affecting Farmers, Truckers, and Motorists.”

https://ago.nebraska.gov/nebraska-attorney-generals-office-issues-safety-alert-regarding-diesel-exhaust-fluid-def-system.

Finally, the EPA is aware that some manufacturers' MY 2027 medium and heavy heavy-duty engine development programs have experienced technical challenges, and a few diesel engine models/families are at risk of not being completed in time for a January 2027 introduction. Consequently, these manufacturers have requested, and the EPA is proposing, NCPs under CAA section 206(g), which would allow these manufacturers to continue to sell their current products while they complete the development of engines compliant with the MY 2027 standards.

C. Summary of the Major Provisions in This Proposal

Truck and engine manufacturers as well as the component supply industry have been preparing for several years to meet the MYs 2027 and later NO

, PM, HC, and CO standards and other requirements finalized in the 2023 Final Rule. Over the past year, most truck and engine manufacturers have communicated to the EPA that they will have a full lineup of products ready to meet the MY 2027 standards. However, some truck and engine manufacturers and the broader industry, including trucking fleets and dealers, have raised concerns with the overall costs of new vehicles resulting from the 2023 Final Rule. In light of concerns from truck and engine manufacturers, the component supply industry, and truck users, trucking fleets, and truck dealerships, the EPA is proposing targeted revisions to certain program elements that are expected to reduce the program costs, while limiting the impact on engine and vehicle manufacturers' significant investments and plans for providing compliant MY 2027 products, and retaining the majority of the projected emissions reductions from the 2023 Final Rule.

In the 2023 Final Rule, the projected costs due to the lengthened emission-related warranty periods were the largest individual contributor to the EPA's projected per-vehicle cost increases, and total program costs. As described in section III.A of this preamble, the EPA is now reconsidering whether the long-term emission reductions projected for longer emission-related warranty periods and the justifications presented in the 2023 Final Rule are reasonable. In consideration of the cost impacts, potential increased purchase price and reduced emission-related warranty coverage options for purchasers, and the associated relatively small emissions impact, the EPA proposes to retain the MY 2026 emission-related warranty periods for MYs 2027 and later heavy-duty engines.

The 2023 Final Rule also lengthened the useful life periods for all heavy-duty engine regulatory classes. The new useful life periods require manufacturers to demonstrate at the time of certification that the emission controls on their MYs 2027 and later engines are durable over the longer periods, and manufacturers are responsible for ensuring that their engines will meet the standards in-use during those useful life periods. In meetings with manufacturers over the past year about their readiness for the MY 2027 program, several manufacturers have noted they can meet the initial certification requirements but expressed concern with the uncertainty of in-use performance of their new technologies over the full useful life for the range of vehicle applications. Based on their technology development work since January 2023, these manufacturers suggest that a shorter useful life would give them more time to evaluate their new technologies in the real world and build more confidence in their ability to meet the MY 2027 standards in-use over the longer useful life periods. As described in section III.B of this preamble, the EPA proposes that the longer useful life periods would apply beginning in MY 2030. This proposed implementation delay is intended to mitigate manufacturer concerns regarding compliance uncertainties for the extended period over which manufacturers have limited data and provide manufacturers with additional lead time to refine emission control systems before the engines are subject to the longer regulatory useful life.

The EPA is also proposing to establish NCPs for certain categories of compression-ignition heavy-duty engines. Under CAA section 206(g), a manufacturer is allowed to produce engines that do not meet the applicable standards “if such manufacturer pays a nonconformance penalty as provided under regulations promulgated by the Administrator after notice and opportunity for public hearing.”

Under existing EPA regulations, the EPA would establish NCPs if three criteria are met: (1) The emission standard in question becomes more difficult to meet; (2) Substantial work must be required to meet the emission standard; and (3) the Agency finds that a manufacturer is likely to be noncomplying for technological reasons (referred to in earlier rules as a “technological laggard”). The EPA believes these criteria have been met for the MY 2027 NO

standards for diesel-fueled engines in the Medium HDE and Heavy HDE regulatory categories and is proposing to establish NCPs for these engine categories as described in section IV of this preamble.

42 U.S.C. 7525(g).

Finally, in the 2023 Final Rule, the EPA codified regulatory requirements to address how manufacturers can certify engines that use DEF in SCR aftertreatment systems. The 2023 Final Rule mandated manufacturers to initiate an engine performance derate, or inducement, to address concerns with DEF quality, DEF supply, and tampering. As described in section V of this preamble, continued operator frustration has motivated the EPA to reassess the Agency's approach to certifying SCR-equipped engines. In this action, the EPA is proposing to replace engine performance derates as a part of SCR-related inducement provisions with visible and/or audible notifications. These proposed amendments for inducements would apply to newly manufactured highway engines and vehicles (

i.e.,

light- and medium-duty vehicles and heavy-duty engines) and in nonroad applications. Under CAA sections 202 and 213, the EPA sets and implements emission standards for new highway and nonroad engines and vehicles throughout their useful life; however, the EPA will be considering new inducement guidance for in-use highway and nonroad diesel engines and vehicles regarding ongoing compliance with such regulations. The EPA is considering whether such guidance might provide recommendations for engine manufacturers to get approval to modify in-use engines, consistent with any provisions the EPA adopts in this rulemaking, without violating prohibitions against tampering and defeat devices.

D. Summary of the Impacts of This Proposal

The EPA updated costs from the 2023 Final Rule Regulatory Impact Analysis (RIA) to reflect the projected change in emission-related warranty costs and operating costs associated with this proposed action. There are no changes to direct manufacturing costs between the proposed action case and the no-action case. The EPA understands that manufacturers have already solidified their technology designs for MY 2027 engines, and the Agency believes that manufacturers are likely to maintain a steady design technology for MY 2028 and 2029 and will not redesign engines in response to the proposed shorter useful life periods for those model years.

As shown in Table I-1, and in more detail in section VI of this preamble, engine manufacturers would see cost savings due to the proposed shorter emission-related warranty period per engine for MY 2027 by regulatory class and fuel type and those savings are likely to be passed on to the vehicle purchaser. See section VI of this preamble and Chapter 3 of the Draft Regulatory Impact Analysis (DRIA) for additional information on the cost impacts of this proposal.

The EPA expects that manufacturers will pass these savings on to purchasers. Thus, the EPA expects purchasers to save up to $37 billion in emission-related warranty savings, including up to $6,000 per diesel vehicle from the warranty savings alone (see Table I-2). Purchasers would also incur additional operating costs, but net cost savings could be as high as $12 billion.

U.S. Environmental Protection Agency. “Amendments and Nonconformance Penalties for Model Year 2027 and Later Heavy-Duty Highway Engines and Amendments to Inducement Provisions for SCR-Equipped Diesel Engines: Draft Regulatory Impact Analysis.” July 2026. EPA-420-D-26-002.

EP14JY26.001

EP14JY26.002

In section VII of this preamble, the EPA presents the combined emissions impacts of the proposed revisions in calendar years 2030, 2040, 2045, and 2055,

which are summarized in Table I-3.

The 2023 Final Rule inventory analysis included calendar years up to 2045. For this proposed rule, the EPA extended analysis years to 2055 to better reflect the impact of fleet turnover further into the future.

EP14JY26.003

The EPA estimates that the 2023 Final Rule, as amended by this proposal, would continue to reduce the onroad heavy-duty NO

inventory in 2055 by about 42 percent—over 260,000 tons of NO

—compared to the 2055 heavy-duty NO

inventory without the 2023 Final Rule. The proposed amendments would retain nearly 90 percent of the NO

reductions originally projected to result from the 2023 Final Rule because the more stringent standards and more representative test cycles finalized in the 2023 Final Rule remain in place.

II. Introduction

A. Background

In January 2023, EPA adopted the 2023 Final Rule, which included new, more stringent emission standards for NO

, PM, HC, and CO for MYs 2027 and later heavy-duty highway engines.

The 2023 Final Rule also changed key provisions of the existing heavy-duty emissions control program, including test procedures, regulatory useful life, emission-related warranty, and other requirements. Additionally, the 2023 Final Rule included changes to maintenance, serviceability, and certain inducement provisions.

88 FR 4296 (Jan. 24, 2023).

B. Response to the 2023 Final Rule

On March 12, 2025, the EPA Administrator announced that the Agency would reevaluate the 2023 Final Rule.

Following this announcement, the EPA heard concerns from truck and engine manufacturers, the component supply industry, and truck users, trucking fleets, and truck dealerships, about the expected cost increases associated with the 2023 Final Rule's changes to the emission-related warranty and regulatory useful life periods (the Agency also received comments from a number of these stakeholders raising concerns with the potential high costs during the development of the 2023 Final Rule). The EPA has also identified several areas where amending specific test procedures and other compliance provisions could support the implementation of the MY 2027 program.

U.S. Environmental Protection Agency. (2025). “EPA Launches Biggest Deregulatory Action in U.S. History.”

https://www.epa.gov/newsreleases/epa-launches-biggest-deregulatory-action-us-history.

Since the 2023 Final Rule was finalized, truck and engine manufacturers as well as the component supply industry have been preparing to meet the MYs 2027 and later NO

, PM, HC, and CO standards and other requirements. Over the past year, most truck and engine manufacturers have communicated to the EPA that they will have a full lineup of products ready to meet the MY 2027 standards. However, some truck and engine manufacturers and the broader industry, including trucking fleets and dealers, have raised concerns with the overall costs of new vehicles resulting from the 2023 Final Rule.

In the 2023 Final Rule, the EPA projected that up to 50 percent of the program costs can be attributed to the longer emission-related warranty periods.

Considering that the length of the emission-related warranty period does not factor into the stringency or feasibility of the standards and would not impact engine manufacturer's investments and production plans, the EPA considers a reduction in the emission-related warranty periods to be a straightforward cost-saving opportunity for engine manufacturers, truck dealerships, and trucking fleets regarding the potential purchase price increases due to the longer emission-related warranty periods.

See the indirect cost calculations presented in Chapter 7.1.2 of the 2023 Final Rule RIA. Control of Air Pollution from New Motor Vehicles: Heavy-Duty Engine and Vehicle Standards Regulatory Impact Analysis. December 2022. EPA-420-R-22-035.

In preparation for meeting the longer useful life periods promulgated under the 2023 Final Rule, most manufacturers have successfully initiated durability testing for MY 2027 certification, which is used to demonstrate that their engines maintain emission performance and meet the standards through the full useful life. However, several manufacturers have expressed concern with increased in-use compliance risk of the longer useful life periods established in the 2023 Final Rule. These manufacturers have noted that they are confident in the performance of their chosen technology pathway to meet the MY 2027 standards over the longer useful life in the laboratory, but they are less certain about how the technology will perform over the longer periods in real-world operation.

Some engine manufacturers have also expressed concerns with the MY 2027 longer useful life as it impacts the near-term use of NO

emission credits under the EPA's averaging, banking, and trading (ABT) program. Relative to credits generated using pre-MY 2027 useful life periods, manufacturers would have to use more credits to apply them to MYs 2027 and later engines, which would accelerate depletion of their credit banks. This accelerated depletion can be especially challenging as manufacturers transition to more stringent standards under a compliance program that also includes new test procedures and other requirements.

In the 2023 Final Rule, the EPA recognized that low-volume specialized vocational vehicles, such as fire trucks, refuse trucks, concrete mixers, emergency vehicles, heavy-haul tractors, and custom-chassis motor homes, can require more extensive redesign to integrate aftertreatment systems needed to meet the MY 2027 standards. In response to concerns that manufacturers needed additional lead time for certain low-volume products, the EPA finalized an interim provision for MYs 2027 through 2029 that allows manufacturers to use NO

emission credits generated under the existing ABT program to certify up to five percent of their production volume of Heavy HDE compliant at pre-MY 2027 requirements.

As manufacturers considered how to apply this five percent production volume allowance for their products, the EPA heard from some manufacturers that they have engines intended for low-volume specialty vehicles, such as refuse trucks, urban buses, motorcoaches, and fire trucks, that would qualify for this allowance, but they do not have sufficient NO

credit balances to use the existing flexibility.

Specifically, this allowance is limited to five percent of a manufacturer's U.S.-directed production volume of Heavy HDE from MYs 2023 through 2025.

The EPA notes that most engine manufacturers are on track to meet the MY 2027 standards. The EPA acknowledges that there was substantial work required to meet these standards, including implementing new technologies and demonstrating compliance with new test procedures.

The technologies manufacturers have chosen for this program range from specific upgrades to the existing emissions control technologies to more comprehensive upgrades that coincide with the development and launch of entirely new engine platforms. The EPA is aware that some manufacturers' MY 2027 Medium HDE and Heavy HDE development programs have experienced technical challenges, and that some diesel engine families are at risk of not being completed in time for a January 2027 introduction (see section IV.A of this preamble). Consequently, these manufacturers have requested that the EPA establish NCPs under CAA section 206(g), which would allow them to continue to sell their current products while they complete the development of the engines compliant with the MY 2027 standards. The EPA is therefore proposing to establish NCPs for diesel-fueled Medium HDE and Heavy HDE for the MY 2027 NO

standards.

The EPA is not reopening MYs 2027 and later test procedures except in the limited ways specifically proposed in section III.E of this preamble, regarding targeted technical amendments to correct or add clarification.

The 2023 Final Rule mandated manufacturers to initiate an engine performance derate, or inducement, to address concerns with DEF quality, DEF supply, and tampering. On August 12, 2025, the Administrator also stated the EPA's intention to further improve operator experience with diesel engines using SCR systems, including assessing whether engine performance derates may no longer be necessary for compliance.

In response to the Administrator's announcement and feedback and concerns that the EPA continues to receive regarding the impact of DEF inducements, the Agency began revisiting the questions and principles discussed in the 2023 Final Rule, including how restrictive inducements should be to provide a reasonable basis for demonstrating that operators would continue to add quality DEF in tanks and not tamper with SCR systems.

U.S. Environmental Protection Agency. (2025). “At Iowa State Fair, EPA Administrator Zeldin Announces Diesel Exhaust Fluid (DEF) Fix.”

https://www.epa.gov/newsreleases/iowa-state-fair-epa-administrator-zeldin-announces-diesel-exhaust-fluid-def-fix.

In addition to the emission-related warranty, useful life, production allowance, and inducement elements from the 2023 Final Rule, the EPA, manufacturers, and other stakeholders have identified several areas where amending the specific test procedures and other compliance provisions could improve the overall accuracy, repeatability, and clarity of the regulations without changing the stringency of the standards.

C. Summary of the Proposal

As noted previously, most engine manufacturers are on track to meet the MY 2027 standards. Therefore, the EPA is not reopening the MYs 2027 and later NO

, PM, HC, and CO standards in this rulemaking.

The EPA is proposing targeted revisions to the emission-related warranty, the regulatory useful life, and the five percent production volume allowance finalized in the 2023 Final Rule. These revisions are intended to reduce costs that could otherwise be passed on to customers as increased purchase prices. In section III.A of this preamble, the EPA describes the Agency's proposal to shorten the emission-related warranty periods for MY 2027s and later engines to the values that apply to MY 2026 and earlier engines. In section III.B of this preamble, the EPA also proposes to delay implementation of the MYs 2027 and later engines' useful life periods to instead start in MY 2030. As such, the current MY 2026 and earlier useful life periods would continue to apply through MY 2029. As described in section III.C of this preamble, the EPA is continuing the existing allowance for manufacturers to produce up to five percent of their U.S.-directed production volume of Heavy HDE that is compliant with pre-2027 requirements, but proposes to remove the requirement to use NO

credits. These targeted revisions are expected to reduce the program costs (see section VI of this preamble), while limiting the impact on manufacturers' current production plans and retaining the majority of the projected emissions reductions from the 2023 Final Rule (see section VII.B of this preamble).

As described in section IV of this preamble, the EPA is also proposing to establish NCPs for certain categories of compression-ignition heavy-duty engines. Specifically, the EPA proposes NCPs for the Medium HDE and Heavy HDE categories.

The EPA is also proposing to replace engine performance derates for SCR-related inducements with visible and/or audible notifications. These new inducement provisions would apply to new heavy-duty highway engines as well as new light-duty and medium-duty vehicles and nonroad diesel engines (see section V of this preamble).

Finally, as described in section III.E of this preamble, the EPA has also identified several additional amendments that would improve the heavy-duty engine programs.

The EPA is proposing specific targeted amendments to correct or add clarification to several test procedures, including the deterioration factor procedure for durability testing, the in-use moving average window procedure for off-cycle testing, the optional engine idle procedure, and the vanadium sublimation procedure for vanadium-based catalyst systems. The EPA is proposing amendments to align the heavy-duty powertrain testing and PM background correction procedures with the light-duty vehicle procedures to limit test burden for manufacturers that certify both light- and heavy-duty products. The EPA is proposing an amendment to extend an allowance for a limited number of qualifying hybrid specialty vehicles to use engines certified to alternative emission standards until MY 2030 under certain conditions. The EPA is also proposing targeted amendments to the onboard diagnostic system requirements that are intended to improve user experiences with MYs 2027 and later emission controls.

It is important to note that while this proposed rule includes a range of specific, targeted amendments to the regulations, the EPA is otherwise not reopening those provisions and is neither proposing nor seeking comment on the underlying standards or program requirements.

D. Legal Authority

Statutory authority for this proposed action comes from the CAA (42 U.S.C. 7401-7675). In this proposed action, the EPA is proposing to amend the emission-related warranty, regulatory useful life, and certain other provisions for MYs 2027 and later heavy-duty engines under CAA sections 202, 203, 206, 207, and 216. Unless provided otherwise by statute, an agency may revise or rescind prior actions so long as it acknowledges the change in position, provides a reasonable explanation for the new position, and considers legitimate reliance interests in the prior position.

See

FDA

Wages & White Lion Invs., L.L.C.,

604 U.S. 542 (2025);

FCC

Fox Television Stations, Inc.,

556 U.S. 502 (2009);

Motor Vehicle Mfrs. Ass'n

State Farm Mut. Auto. Ins. Co.

, 463 U.S. 29 (1983);

Clean Air Council

Pruitt,

862 F.3d 1, 8 (D.C. Cir. 2017) (“Agencies obviously have broad discretion to reconsider regulations at any time”).

E. Reliance Interests

The EPA understands that companies have previously planned for the MY 2027 start date for the MYs 2027 and later heavy-duty engine emission standards and related program requirements.

The EPA acknowledges that this proposal would, if finalized, change the Agency's previous assessments in the 2023 Final Rule with respect to the emission-related warranty, regulatory useful life, and other program elements. The EPA notes that this proposal does not change the emission standards and related test cycles established in the 2023 Final Rule for MY 2027 and later. As explained throughout this preamble, the EPA believes this change in position is necessary. The EPA believes that the revisions proposed in this action relieve obligations in a manner that promotes compliance and cost savings without undermining existing investments in compliance. Nevertheless, the EPA requests comment on whether regulated parties have any significant reliance interests with respect to the changes in emission-related warranty, regulatory useful life, and other program elements contained in this proposal. (C-1).

The EPA further understands that other interested parties may have relied on the MY 2027 heavy-duty program for independent purposes, including compliance with relevant National Ambient Air Quality Standards (NAAQS) and related planning obligations, among others. The EPA believes that the relatively small foregone emissions reductions involved in this proposed revision, coupled with the relatively short amount of time that has passed since promulgation of the 2023 Final Rule, means that such interests do not supersede the Agency's obligation to ensure that program elements are appropriate and reflect technical and market realities. Moreover, the EPA notes that emissions from mobile sources are just one consideration among many involved in planning to attain the NAAQS and related obligations. Nevertheless, the EPA requests comment on such reliance interests and how such interests should be taken into account in any final action on this proposal. (C-2).

The EPA requests comment on the nature and extent of any other reliance interests that may arise from this proposed action and is committed to assessing any such interests, determining whether they are significant, and weighing such interests against competing rationales, as required by law. (C-3).

III. Proposed Compliance Provisions and Test Procedures

The EPA is proposing targeted revisions to the 2023 Final Rule to aid in the implementation of the MYs 2027 and later HD engine criteria pollutant emissions program, reduce the cost of the program, and improve operator experiences. The proposed changes include continuing the MY 2026 and earlier emission-related warranty periods for MYs 2027 and later engines, delaying the implementation of the MY 2027 useful life periods by three years, and targeted revisions to ABT credit flexibilities. The EPA is also proposing discrete revisions to correct and/or clarify certain identified regulatory provisions. This section III describes the proposed revisions.

A. Emission-Related Warranty

The emission-related warranty period is the period over which CAA section 207 requires an engine manufacturer to warrant to a purchaser that: (1) the engine is designed, built, and equipped so as to conform with applicable regulations under CAA section 202 and (2) is free from defects in materials or workmanship which would cause the engine not to conform with applicable regulations for the warranty period. If an emission-related component fails during the regulatory emission-related warranty period, the manufacturer is required to pay for the cost of repair or replacement. A manufacturer's general emission-related warranty responsibilities are currently set out in 40 CFR 1068.115. Note that while an emission-related warranty provides protection to the owner against emission-related repair costs during the warranty period, the owner is responsible for properly maintaining the engine (40 CFR 1068.110(e)), and the manufacturer may deny warranty claims for failures caused by the owner's or operator's improper maintenance or use (40 CFR 1068.115(a)).

In the 2023 Final Rule, the EPA finalized increases in the emission-related warranty periods for MYs 2027 and later engines, as shown in Table III-1, which relied on three primary justifications.

First, the EPA expected longer emission-related warranty periods would lead owners to continue to maintain their engines and vehicles over a longer period of time, which would ensure longer-term benefits of emission controls. Second, the EPA

noted that manufacturers may be more incentivized to simplify complex components to reduce in-use failures if they were responsible for repairs over longer periods. Finally, the EPA projected that longer warranty periods would allow manufacturers to have access to better defect information over a period of time more consistent with engine useful life. Manufacturers are currently required to track and report defects to the EPA under the defect reporting provisions of 40 CFR part 1068. Under 40 CFR 1068.501(b), manufacturers investigate possible defects whenever a warranty claim is submitted for a component and can monitor defect information from dealers and repair shops that perform those warranty repair services. After the emission-related warranty period ends, a manufacturer would not necessarily know about these defects, since repair facilities are less likely to be in contact with the manufacturers and less likely to use original equipment manufacturer (OEM) parts.

88 FR at 4364.

EP14JY26.004

The EPA is reconsidering the longer emission-related warranty periods in light of new information on costs and potential impact on purchase price. The EPA proposes that the relatively small long-term emission reductions projected for longer emission-related warranty periods (see section VII of this preamble) and the uncertain long-term improvements to component quality and serviceability that the Agency previously used to justify the longer warranty periods do not outweigh the costs and potential impact on purchase price.

The 2023 Final Rule estimated the increased emission-related warranty costs per year for the vehicle regulatory classes represented in EPA's Motor Vehicle Emission Simulator (MOVES) model,

which generally match the primary intended service classes of vehicles in EPA's regulations. The EPA also estimated the age at which the range of MOVES heavy-duty vehicle regulatory classes would reach their emission-related warranty limit based on their projected mileage accumulation.

The EPA inventory analysis for this proposal was performed using a regulatory version of the Agency's MOVES model, known as MOVES5.R2, which was derived from the latest public version (MOVES5) with the updates described in Chapter 2.2 of the DRIA for this proposed rule.

These costs were presented in Tables 7-14 through 7-17 of the 2023 Final Rule RIA. See Control of Air Pollution from New Motor Vehicles: Heavy-Duty Engine and Vehicle Standards, Regulatory Impact Analysis. December 2022. EPA-420-R-22-035.

In the 2023 Final Rule, the EPA projected significant NO

emission reductions in calendar year 2045 from vehicles subject to the MYs 2027 and later standards and other requirements.

The NO

emission reductions attributable to the lengthened emission-related warranty periods were relatively small compared to the reductions due to the more stringent standards and more representative test cycles also finalized under that program. While the EPA did not separately present the contributions from each program element in the 2023 Final Rule, the Agency is updating the analysis in this proposed rule to include the contributions of the program elements we are proposing to revise (see section VII of this preamble). By removing the longer emission-related warranty periods, the EPA projects NO

emissions would increase by approximately 36,000 tons in calendar year 2055. In the same year, the EPA estimates that the remainder of the 2023 Final Rule program is projected to reduce NO

emissions by approximately 260,000 tons relative to the standards and requirements in place for MY 2026 and earlier engines. The proposed changes to emission-related warranty periods would reduce the NO

emission benefits estimated in the 2023 Final Rule by 12 percent in calendar year 2055.

See Table VI-1 of the 2023 Final Rule (88 FR 4418, Jan. 24, 2023).

In response to the new emission standards for the MY 2027 program, manufacturers have added technology that qualifies as emission-related components covered under warranty. For any new emission-related components, manufacturers project purchase price adjustments to cover anticipated warranty costs, which would have resulted in a higher warranty cost pursuant to the 2023 Final Rule. By lengthening emission-related warranty periods in addition to adopting new emission standards for the MY 2027 program, the 2023 Final Rule has required manufacturers to adjust their MYs 2027 and later warranty costs to cover the new technology, as well as costs to cover the emission-related components already on their engines for longer coverage periods.

In the 2023 Final Rule, emission-related warranty costs were the largest individual contributor to the per-vehicle cost increases and the total program costs. Since the 2023 Final Rule, the EPA has met with manufacturers and has learned that the emission-related warranty costs could be significantly higher for certain applications. In the case of the Heavy HDE category, OEMs have shared that the cost increase for just the emission-related warranty could exceed $15,000 for some engines intended for use in low-mileage vocational vehicles for which the engine would be covered by warranty until the 10 year limit.

Commenters on the proposal to the 2023 Final Rule, including engine manufacturers, suppliers, purchasers, and industry trade associations, stated that emission-related warranty costs are generally passed on to customers in the vehicle purchase price, and they noted that such an increase could impact vehicle sales.

This is consistent with the 2023 Final Rule emission-related warranty cost analysis that estimated warranty costs are approximately $1,000 per year.

See sections 4 and 25 of the EPA's summary and response to comments on the proposal to the 2023 Final Rule. “Control of Air Pollution from New Motor Vehicles: Heavy-Duty Engine and Vehicle Standards Response to Comments.” EPA-420-R-22-036. December 2022.

Longer emission-related warranties may not align with the preferences or usage patterns of all purchasers. In comments on the proposal to the 2023 Final Rule, American Trucking Associations (ATA) noted that certain fleets purchase extended warranties based upon the unique business models and financial means of each company.

ATA stated that fleets with short turnover cycles for their vehicles should not be required to purchase warranties that exceed their mileage needs and that such a requirement may have the unintended consequence of some fleets retaining their current trucks longer. In response to this comment in the 2023 Final Rule, the EPA noted that, to the extent that fleets currently base their shorter turnover cycle on the EPA emission-related warranty periods, the Agency expected those fleets to consider adjusting their current business model to accommodate the longer warranties in the 2023 Final Rule.

In reconsidering the longer emission-related warranty periods, the EPA acknowledges that the shorter, pre-2026 warranty periods would allow these companies to continue to opt for additional warranty coverage if it meets their specific operational requirements, which would reduce the influence of the warranty as a factor a business might consider when deciding whether to purchase a new vehicle.

Comments of the American Trucking Associations: On the Control of Air Pollution from New Motor Vehicles: Heavy-Duty Engine and Vehicle Standards Proposed Rule. EPA-HQ-OAR-2019-0055-1326-A1. Pages 11-17.

See section 18.3 of the EPA's summary and response to comments on the proposal to the 2023 Final Rule. “Control of Air Pollution from New Motor Vehicles: Heavy-Duty Engine and Vehicle Standards Response to Comments.” December 2022. EPA-420-R-22-036. p. 1147.

While the required emission-related warranty period for manufacturers is proposed to be shortened, the post-warranty period for which owners would be responsible for emission-related repairs would be lengthened. The increased emission-related repair costs associated with the proposed warranty periods are reflected in the operating costs of the EPA's cost analysis. See Chapter 3 of the DRIA for this proposed rule.

In consideration of the cost impacts, potential increased purchase price and reduced warranty coverage options for purchasers, and the associated, relatively small emissions impact compared to the full scope of the 2023 Final Rule, the EPA proposes to retain the MY 2026 emission-related warranty periods for MYs 2027 and later. This proposal is based on the recognition that maintaining the much longer emission-related warranty for MYs 2027 and later will significantly increase the manufacturer costs and consumer prices for a relatively small emissions benefit. The EPA believes that the emission-related warranty periods in place for MY 2026 and earlier engines would continue to ensure that the regulated industry would design and warrant robust emission control technologies while allowing commercial truck purchasers to make their own decisions on the need to purchase extended warranties (which many purchasers do today).

B. Regulatory Useful Life Periods

Useful life represents the period over which emission standards apply to certified engines, and, practically, any difference between the regulatory useful life and the generally longer operational life of in-use engines represents miles and years of operation without an assurance that emission standards will continue to be met. For the EPA's heavy-duty highway engine program, useful life periods vary by engine class to reflect the different lengths of their estimated operational lives. As described in the 2023 Final Rule, the EPA considers operational life to be the average mileage at rebuild for compression-ignition engines (

i.e.,

engines typically fueled by diesel or natural gas) and the average mileage at replacement for spark-ignition engines (

i.e.,

engines typically fueled by gasoline or liquefied petroleum gas).

EP14JY26.005

The MYs 2026 and earlier useful life periods have been in place since 2004, and manufacturers have had decades to refine their durability demonstrations and understand the in-use durability of their emission controls over those periods. The new useful life periods require manufacturers to demonstrate at the time of certification that the emission controls on their MYs 2027 and later engines are durable over the longer periods. Additionally, the new emission standards, which the EPA is not reopening in this proposed rule, will apply to the applicable useful life periods, and manufacturers are responsible for ensuring that their engines will meet the standards in-use over those useful life periods.

The EPA included flexibilities in the 2023 Final Rule to limit additional certification test burden and in-use uncertainty due to the longer useful life periods. The EPA finalized a new durability test procedure that allows manufacturers to accelerate the aging of their emission controls to capture the additional time without a proportionally increased test burden. Manufacturers are actively applying the accelerated aging procedures for their MY 2027 engine certification and the EPA continues to receive data on those activities.

In terms of in-use uncertainty, the EPA finalized a 15 milligrams per brake horsepower-hour (mg/hp·hr) in-use compliance margin for Medium HDE and Heavy HDE for in-use testing of engines to the laboratory-based duty cycles and off-cycle test procedures,

and also added an accuracy margin to address uncertainty in the portable emissions measurement equipment used for onroad in-use testing.

In section III.E.1 of this preamble, the EPA proposes clarifications and corrections resulting from EPA and manufacturer experiences in applying these new procedures.

88 FR 4334.

88 FR 4353.

In meetings with manufacturers over the past year about their readiness for the MY 2027 program, some

manufacturers have indicated that they are on target to meet the requirements of the longer useful life periods, including in-use performance. Other manufacturers noted they can potentially meet the certification requirements but expressed concern with the uncertainty of in-use performance of their new technologies over the full useful life for the range of vehicle applications. Manufacturers suggest that a shorter useful life would give them more time to evaluate their new technologies in the real world and more confidence in the in-use durability over the longer useful life periods.

In response to industry feedback and the credible information shared about their technological readiness, the EPA proposes a three-year delay in the implementation of the increased useful life periods for heavy-duty engines shown in Table III-2. Specifically, the EPA proposes that the new useful life periods would apply beginning in MY 2030. This proposed implementation delay is intended to mitigate manufacturer concerns regarding compliance uncertainties for the extended period over which manufacturers have limited data and provide manufacturers with additional lead time to refine emission control systems before the engines are subject to the longer regulatory useful life.

Delaying the longer regulatory useful life periods would provide heavy-duty engine manufacturers with time to collect durability data on their in-use engines with the new technologies without having to manage the risks and burdens of a potential engine recall due to exceeding the emission standards during the longer useful life periods. Manufacturers have spent four or more years developing and testing engines, both in the laboratory and in vehicles with their low volume pre-production engines, to ensure that their engines will be durable and able to meet the standards for the longer useful life. However, since this development has been limited to pre-production engines, no company has real-world data on MY 2027 production hardware over the range of applications in which these engines may be used. Manufacturers have thorough development processes to test their products under conditions that reflect some of the most challenging real-world operations, and there is additional experience which will be gained with production engines in the real-world with the diversity of the engines' applications. Under the proposed delay of the longer regulatory useful life periods to MY 2030, manufacturers will be able to gain this real-world, in-use experience through three years of production engines subject to the MY 2027 standards. The proposed delay would provide additional lead time during which manufacturers can refine their engines and emission controls over the expanded operating conditions covered in the MYs 2027 and later program at the current useful life periods. This is intended to increase manufacturers' confidence that their products will comply with the standards for the duration of the longer useful life periods.

As shown in section VII of this preamble and Chapter 2 of the DRIA for this proposed rule, the EPA projects that the proposed delay in the lengthened useful life periods would have a minimal impact on the emissions inventory. To estimate the emissions inventory impacts, the EPA applied the shorter useful life periods for MY 2027 through MY 2029. The EPA believes this approach to estimating emissions impacts is conservative, because the timing of this proposed rule may limit the changes manufacturers would be willing to make to their engines prior to the start of production for MY 2027. The EPA expects most manufacturers will likely produce the same engines they have been preparing to manufacture for MY 2027; in fact, there is the potential that more new vehicles will be available for sale that would increase the probability that consumers will purchase newer vehicles that have a better emissions profile relative to their existing vehicles. In addition, because the proposal does not alter the emission standards, manufacturers will be motivated to gain as much real-world experience with the technology needed to meet the standards at the longer useful life. Therefore, the EPA does not expect engines to be less durable under this proposed change.

Based on discussions with individual engine manufacturers, the EPA expects the manufacturers that are currently prepared to certify engines meeting the longer useful life periods would equally benefit from the longer lead time to design their engines to address any uncertainties they may have regarding their in-use compliance. However, the EPA recognizes that the Agency must consider, and correspondingly propose to address, discrepancies that would result relating to durability testing that manufacturers may have already completed at the longer useful life periods. The EPA is proposing two methods under a new interim provision in 40 CFR 1036.150, which would apply to MY 2027 through MY 2029, depending on whether the manufacturer included an emissions test point at the useful life periods that apply to MYs 2026 and earlier (

e.g.,

equivalent to 435,000 miles for Heavy HDE) or if they only conducted the test to the full MYs 2027 and later useful life (

e.g.,

equivalent to 650,000 miles for Heavy HDE). If available, the EPA is proposing under this interim provision that manufacturers must use test data from the point that is equivalent to the useful life periods that apply to MYs 2026 and earlier. If manufacturers did not collect data at that point, the EPA would allow manufacturers to interpolate their data to the shorter useful life periods that apply to MYs 2026 and earlier using the data at the low-hour and longer useful life test points. The EPA requests comment on this interim provision. (C-4).

With this proposed change to useful life, there would likely be a reduction in manufacturers' costs. As stated above, the EPA does not expect manufacturers who have products ready for MY 2027 to change their emissions control technology plans for the MYs 2027 through 2029 period because of this proposed change. However, manufacturers that have not completed their durability demonstration for the longer useful life may choose to stop the demonstration at the current useful life values, which could result in a small reduction in testing costs that manufacturers could pass on to their customers. For this proposal, the EPA has taken a conservative approach to estimating the cost impacts of the proposed change to useful life by projecting no change in costs. The EPA requests comments on the cost savings from this provision. (C-5). See section VI of this preamble and Chapter 3 of the DRIA for this proposed rule for more details on the estimated cost impacts of the proposed change to useful life.

Finally, the EPA notes that useful life also plays a role in manufacturers' credit balances for those companies participating in the ABT program.

Useful life is a variable in the equation to calculate NO

emission credits (see 40 CFR 1036.705(b)), and credits earned and used are proportional to useful life. For example, unless a manufacturer were to comply with the longer useful life periods early, credit, earned from a 150 mg NO

/hp·hr Heavy HDE in MY 2022 to 2026 (

i.e.,

50 mg/hp·hr below the standard) would be generated using a useful life of 435,000 miles, and if the credits were used to comply with an 65 mg NO

/hp·hr Heavy HDE (

i.e.,

30 mg/hp·hr above the standard) for MY 2027, the credits would be applied based on

a useful life of 650,000 miles. Therefore, for Heavy HDE, credits would be used at a 1.49 times greater rate than earned for a useful life of 650,000 miles, for the same difference in mg of NO

per hp·hr between the Family Emission Limit (FEL) and the standard. Delaying the increase in useful life periods will positively impact the NO

credits manufacturers have accrued from selling MY 2022 and later engines that are over compliant with the pre-MY 2027 NO

standard. The resulting lower credit usage rate for MYs 2027 and later will allow manufacturers to extend the use of credits already earned for a longer period and/or apply those credits to a larger volume of engines during the transition to lower standards.

See Chapter 1 of the DRIA for this proposed rule.

In summary, the proposed delay for implementing the longer useful life periods, and the associated reduction in credit usage that would result, would afford manufacturers additional time to gain experience with new technologies and test procedures that will begin to apply in MY 2027. This additional time could allow for refining designs, optimizing performance, and ensuring reliability under a wide range of in-use operational conditions. Manufacturers could leverage this extended lead time to conduct further testing and validation, ensuring that innovations are not only compliant but also robust when the useful life is increased in MY 2030 under the proposed delay. By proposing this additional time, the EPA aims to balance the objectives of stringent emission standards with pragmatic considerations of industry readiness and corresponding uncertainties.

Finally, the EPA requests comment on indefinitely keeping the MY 2026 and earlier regulatory useful life periods for MYs 2027 and later. (C-6). Chapter 2 of the DRIA for this proposed rule presents a scenario that projects the emissions impacts of never extending the useful life periods. For this scenario, the EPA has not estimated cost savings due to changes in the emissions control technology. However, as discussed in Chapter 3 of the DRIA for this proposed rule, the EPA would expect manufacturers to change the design of the emissions controls if the Agency were to finalize indefinitely keeping the MY 2026 and earlier regulatory useful life periods for MYs 2027 and later. The EPA understands stakeholders could potentially have concerns that products meeting the longer useful life periods have not yet entered the market and thus there is still some uncertainty in the feasibility of the standards through the longer useful life periods for all real-world applications.

C. Production Volume Allowance

In the 2023 Final Rule, the EPA adopted an interim production volume allowance for MYs 2027 through 2029 in 40 CFR 1036.150(k) that allows manufacturers to use NO

emission credits to certify a limited volume of Heavy HDE compliant with pre-MY 2027 requirements.

To aid in implementation, the EPA chose not to limit the production volume allowance to specific applications of Heavy HDE but did limit it to no more than five percent of the average U.S.-directed production volumes of Heavy HDE over three MYs. In the 2023 Final Rule, the EPA noted that such an allowance from the MY 2027 criteria pollutant standards may be necessary to provide engine and vehicle manufacturers with additional lead time and flexibility to redesign some low-volume vehicles to accommodate the technologies needed to meet the more stringent engine emission standards.

Engines certified under this production volume allowance would meet the pre-MY 2027 engine provisions of 40 CFR part 86, subpart A.

The EPA proposes to modify 40 CFR 1036.150(k) to not require the use of NO

credits in MYs 2027 through 2029 for engines certified under the interim production volume allowance. Under this proposal, a manufacturer certifying up to five percent of its Heavy HDE U.S.-directed production volumes under 40 CFR 1036.150(k) would not be required to use NO

credits to offset the negative credits associated with those engines. All other conditions of 40 CFR 1036.150(k) would remain in effect and the EPA is not proposing changes to those provisions in this proposal, including the five percent cap based on the MYs 2023 through 2025 U.S.-directed production volume average, the limitation to Heavy HDE, certification to the MY 2026 requirements in 40 CFR part 86, subpart A, the carryover-family requirement, declaration of a NO

FEL at or below the standard set out in 40 CFR 86.007-11, and applicable reporting and recordkeeping.

The EPA is proposing this change because the Agency has heard from multiple engine manufacturers that they do not have sufficient NO

credit balances to use the existing flexibility that was designed to provide additional lead time for low-volume vehicles applications that face unique engineering and packaging constraints.

Specialized vocational vehicles, such as fire trucks, refuse trucks, concrete mixers, emergency vehicles, heavy-haul tractors, urban buses, and custom-chassis motor homes can require more extensive redesign to integrate aftertreatment systems needed to meet the MY 2027 standards. In the 2023 Final Rule, the EPA received comments from manufacturers and vehicle builders who indicated that, for these applications, the timing and complexity of changes, especially where aftertreatment may need to be mounted off the frame rails or where space constraints are severe, can be more challenging than for mainstream linehaul tractor configurations.

See Chapter 1 of the DRIA for this proposed rule.

In the past year, the EPA has heard from a number of engine manufacturers and low-volume vehicle manufacturers that the challenges discussed in the 2023 Final Rule continue to exist, and additional time is needed for some specialized vocational vehicles. However, not all engine manufacturers have NO

credits available to make use of the flexibility. Removing the requirement to use NO

credits, within the other existing requirements including the five percent cap and limitation to Heavy HDE, would offer a practical, near-term flexibility to bridge these constraints without expanding the duration or volume of the allowance. In addition, due to the nature of the heavy-duty industry that has a limited number of engine manufacturers, the industry has made little use of the trading provisions in the ABT program. As described in Chapter 1 of the DRIA for this proposed rule, since MY 2012, only four NO

emissions credit trades have occurred. The absence of trading and the limited banks of NO

credits necessary for utilizing production volume allowance reduces its effectiveness as a flexibility and threatens the continued availability of the low volume HD vehicles for which the provision was intended to address. This situation further supports the proposed change to eliminate the requirement of using NO

credits to access the interim production volume allowance.

The proposed change affects only up to five percent of U.S.-directed Heavy HDE production in MYs 2027 to 2029 and would have a small impact on the emissions inventory. All other engines would remain subject to the MY 2027 standards, and engines certified under the allowance would still meet the MY 2026 requirements. This approach preserves the core emissions benefits of the 2023 Final Rule while addressing discrete implementation needs. See section VII of this preamble and Chapter 2 of the DRIA for this proposed rule for more details on the estimated emissions impacts of the proposed change to 40 CFR 1036.150(k).

The EPA requests comment on the proposed change to the MYs 2027-2029 allowance under 40 CFR 1036.150(k), including whether it appropriately balances the implementation needs and protection of emissions benefits. (C-7). The EPA also requests comment on whether this flexibility should apply to fewer MYs (

e.g.,

only MY 2027, or MYs 2027 and 2028), or more MYs (

e.g.,

also MY 2030 or beyond). The EPA also requests comment on whether the five percent cap should be lower or higher, and if the cap should remain constant or phase down over time. See section III.D for other credit flexibilities the EPA may consider to lower compliance costs and make implementation of standards smoother.

D. Credit Flexibilities

In the 2023 Final Rule, the EPA established NO

FEL caps at 65 mg/hp·hr for MYs 2027 through 2030 and 50 mg/hp·hr for MYs 2031 and later. This prevents a manufacturer from certifying an engine with NO

emissions greater than the FEL cap, even if a manufacturer has enough emission credits to continue producing a type of engine that is greater than the FEL caps.

In addition to establishing FEL caps, the 2023 Final Rule also restricts NO

credits that can be used in MYs 2027 and later to those credits earned beginning in MY 2022. NO

credits from MYs 2021 and earlier can be used through MY 2026, but not beyond. The 2023 Final Rule also retained the EPA's historical approach of separate averaging sets based on primary intended service class: heavy-duty spark-ignition engines (Spark-Ignition HDE), light heavy-duty engines (Light HDE), Medium HDE, and Heavy HDE. NO

emissions ABT is only allowed within each of these averaging sets; that is, no emissions can be averaged or traded between the categories. Additional information on the 2023 Final Rule ABT program provisions, as well as the industry NO

emissions credit banks through MY 2024, can be found in Chapter 1 of the DRIA for this proposal.

In the past year, multiple regulated entities requested the EPA modify the NO

ABT program for MYs 2027 and later. This section III.D summarizes and solicits comment on whether to increase the NO

FEL cap, allow the use of NO

credits across engine service classes (

i.e.,

Spark-Ignition HDE, Light HDE, Medium HDE, and Heavy HDE), and allow the use of NO

emission credits generated prior to MY 2022.

1. NO

FEL Cap

The EPA recognizes that increasing the FEL cap, even in the short-term, could reduce the economic costs of the requirements, potentially without significant foregone emissions reductions. However, the EPA also recognizes that doing so would be a change in Agency position. In the 2023 Final Rule, the NO

FEL cap was set below the existing standard of 200 mg/hp·hr to avoid competitive disruptions and ensure that the majority of new engines introduced into commerce would include updated emissions control technologies compared to the emissions control technologies manufacturers were using to meet the existing standards.

The EPA also explained in the 2023 Final Rule that this was consistent with the Agency's past practice when issuing rules for heavy-duty onroad engines or nonroad engines in which there was a substantial (

i.e.,

greater than 50 percent) difference between the numeric levels of the existing and new standards).

Comments received on the 2023 Final Rule from manufacturers suggested that the EPA should set an FEL cap at a level between 50 and 100 mg/hp·hr, stating that such a cap would help to prevent competitive disruptions (

i.e.,

require all manufactures to make improvements in their emissions control technologies). In the 2023 Final Rule, the EPA explained that the specific numeric levels of the final FEL caps were selected to balance several factors. The EPA stated that these factors include providing sufficient assurance that lower-emissions technologies will be introduced in a timely manner, which was consistent with the Agency's past practice,

and providing manufacturers with flexibility in their product planning or assurance against unforeseen emissions-related problems that may arise. In the 2023 Final Rule, the EPA explained that the FEL cap of 65 mg/hp·hr selected for the early years of the program (

i.e.,

MYs 2027 through 2030) placed more emphasis on providing manufacturers flexibility and assurance against unforeseen emissions control issues to ensure a smooth transition to the new standards and avoid market disruptions, as a smooth transition in the early years of the program helps ensure the public health benefits of the final program by avoiding delayed emissions reductions due to slower fleet turnover than may occur without the flexibility of the ABT program in the 2023 Final Rule. In the 2023 Final Rule, the EPA stated that the FEL cap of 50 mg/hp·hr selected for the later years of the program (

i.e.,

MYs 2031 and later), placed more emphasis on ensuring continued improvements in the emissions control technologies installed on new engines.

88 FR 4296 at 4393.

69 FR 38997 (June 29, 2004); 66 FR 5111 (Jan. 18, 2001).

See 69 FR 38997 (June 29, 2004).

At the time of this proposed rule, engine manufacturers are generally making progress toward adopting new emissions control technologies to meet the standards; however, one engine manufacturer has indicated that an increased FEL cap of 200 mg/hp·hr for up to two MYs could help them in the transition to meeting the MYs 2027 and later standards. With respect to potential emissions impact, the EPA notes that a company would only be able to make use of a higher FEL cap if the company had NO

credits available. See Chapter 1 of the DRIA for this proposal for a discussion of emissions ABT and data regarding engine manufacturers' NO

credit balances.

The EPA notes that if a higher NO

FEL cap were established in the final rule of this proposal, this would be optional and would not require any company to make use of the higher FEL cap. The EPA notes that some engine manufacturers may have concerns with the change in the competitive landscape from an increase in the FEL cap, as a higher FEL cap may allow a firm to sell an engine with a less-effective emissions control technology at a lower cost.

The EPA requests comment on increasing the NO

FEL cap to a level greater than 65 mg/hp·hr, but less than or equal to the MY 2026 standard of 200 mg/hp·hr. (C-8) The EPA also requests comment on the number of MYs over which a higher FEL cap should apply, including whether it is appropriate to not include an end date for a higher FEL cap. For example, the EPA may consider a higher FEL cap for a few MYs during the transition to the lower NO

standard followed by a return to 65 mg/hp·hr, followed by the 50 mg/hp·hr cap for MYs 2031 and later, or an alternative level, or an increase in the FEL cap without a specified MY end date. The EPA requests comment on the possible economic and emissions impacts of allowing older credits to be used in the MYs 2027 and later program. The EPA also requests comment on any reliance interest engine manufacturers or others may have, including the specific reliance concerns and available data supporting a reliance argument.

See section II.E of this preamble.

2. Use of Pre-MY 2022 Emission Credits

The EPA recognizes that allowing the use of pre-MY 2022 emission credits would be a change in Agency position. The 2023 Final Rule restricted credit use to those generated in MYs 2022 and later, noting that a majority of the existing credit balances were accumulated before MY 2009 to create flexibility in transitioning to the MYs 2007-2010 standards and those credits were generated from engines without modern emissions control technologies.

Additionally, to account for the addition of a new low-load test cycle (LLC), standard and moving average window in-use test procedures and emission standards for MYs 2027 and later, and the resulting difference in emission controls, the 2023 Final Rule applies up to a 40 percent discount on NO

credits earned between MYs 2022 and 2026 when those credits are used in the MYs 2027 and later program. By applying these restrictions, the 2023 Final Rule expected to incentivize manufacturers to maximize their development and introduction of the best available emissions control technologies ahead of when they are required to do so in MY 2027, instead of relying on older credits.

88 FR 4296 at 4395.

As noted previously, at the time of this proposed rule, engine manufacturers are generally making progress toward adopting new emissions control technologies to meet the standards. The EPA's proposals to expand access to the existing transitional production volume allowance for Heavy HDE and establish NCPs for Medium HDE and Heavy HDE are also expected to help manufacturers needing additional time to transition to the MYs 2027 and later standards for those engine categories. However, some manufacturers have expressed an interest in using the pre-MY 2022 credits. Manufacturers have indicated the pre-MY 2022 credits could be used as an alternative to the use of NCPs, or could be used for other product planning purposes, such as to increase the FEL of a MY 2027 or later engine family, and/or to increase the number of engines produced in the future at a given FEL above the standard.

The EPA notes that if the use of pre-MY 2022 credits were allowed in the final rule of this proposal, this would be optional. The EPA notes that some engine manufacturers may have concerns with changing this credit use provision due to potential changes in the competitive landscape from the use of older credits, as a firm may now be able to sell an engine with a less-effective emissions control technology at a lower cost.

The EPA requests comment on whether, and how, banked NO

emission credits earned prior to MY 2022 should be allowed for use in MYs 2027 and later. (C-9). As shown in Chapter 1 of the DRIA for this proposed rule, no NO

emission credits were generated in MYs 2010-2021, so allowing pre-MY 2022 credits would in practice be credits generated in MYs 2004-2009. The EPA notes that some of the MY 2004 to MY 2009 engines may be nearing the end of their practical use (or have been retired or scrapped) and on average are accumulating fewer annual vehicle miles relative to MYs 2027 and later engines.

Furthermore, as noted previously, credits generated from MYs 2004 to 2009 engines were generated by engines without modern emission controls and relative to different test procedures and standards. A discount of up to 40 percent is applied when using credits earned during MYs 2022-2026 for MYs 2027 and later, and the EPA requests comment on an appropriate discount that could be applied to credits earned from the MYs 2004 to 2009 engines for use in MYs 2027 and later. Finally, the EPA requests comment on the possible economic and emissions impacts of allowing these older NO

credits to be used in the MYs 2027 and later program. The EPA also requests comment on any reliance interest engine manufacturers or others may have, including the specific reliance concerns and available data supporting a reliance argument.

As shown in the relative mileage accumulation rates of Figure 6-4 in the MOVES Technical Report “Population and Activity of Onroad Vehicles in MOVES5” (EPA-420-R-24-019, November 2024), Single Unit Trucks and Combination Trucks that are 20 years or older are predicted to travel 75 percent fewer miles than their new counterparts.

https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P101CUN7.pdf.

See section II.E of this preamble.

3. Credit Use Across Averaging Sets

The EPA recognizes that allowing the use of emission credits across averaging sets would be a change in Agency position. The EPA first established the NO

emission credit banking and trading flexibilities for heavy-duty engines in 1990, which expanded the averaging program already in place for those engines.

That 1990 rule continued the use of averaging sets to denote the heavy-duty engine subcategory within which credits could be averaged, banked, or traded. The EPA established a single averaging set for all Spark-Ignition HDE, referred to as “Otto-cycle engines” at the time, and light, medium, and heavy averaging sets for compression-ignition HDE (“diesel cycle engines”). That 1990 rule expanded the existing averaging set restrictions to include trading and banking such that emission credit averaging and trading are not permitted between averaging sets and banked credits can only be used in the averaging set in which they were generated.

In the proposal to that 1990 rule,

the EPA justified the averaging set restrictions by noting concern that cross-subclass credit exchanges would affect the environment and competition. In the 1990 final rule, the EPA noted that commenters requesting the Agency permit such an approach did not offer solutions that would adequately address the concerns discussed in the proposed rule.

55 FR 30534 (July 26, 1990).

Id. at 30588.

54 FR 22670 (May 25, 1989).

The 2023 Final Rule retained the existing averaging sets (

i.e.,

Spark-Ignition HDE, Light HDE, Medium HDE, Heavy HDE) and allowed ABT only within those averaging sets. The 2023 Final Rule noted the approach was consistent with the Agency's historical approach, which avoids creating unfair competitive advantages or environmental risks due to credit inconsistency.

The 2023 Final Rule did include one temporary exception allowing credits generated by MY 2026 Heavy HDE to be used for certifying Medium HDE after applying a 10 percent discount.

This exception was intended as a transitional credit pathway to encourage the early introduction of lower-emitting engines as manufacturers prepared for the new MYs 2027 and later program.

See 55 FR 30585 (July 26, 1990); 66 FR 5002 (Jan. 18, 2001).

See the 2026 service class pull ahead transitional credit pathway under 40 CFR 1036.150(a)(4).

See 88 FR 4296 at 4397 and Attachment 5 to the 2023 Final Rule's Stakeholder Meeting Log in the docket for that rule:

https://www.regulations.gov/document/EPA-HQ-OAR-2019-0055-2895

for a discussion of the 10 percent cross-service class credit discount.

The EPA requests comment on whether and how emission credits could be utilized between engine service classes. (C-10). As included in the 2023 Final Rule, specific limited allowances for cross-averaging set transfers have been allowed to encourage the early introduction of lower-emitting engines during the transition to new emission standards. These flexibilities allow manufacturers, for a limited period, to focus their investments on a specific service class

and then use those emission credits in other services classes. If the EPA were to finalize an allowance for cross-averaging set trading, it could similarly allow manufacturers to prioritize their resources.

If the EPA were to allow cross-averaging set NO

credit transfers, the Agency requests comment on whether a credit discount should be applied, the value of such a discount, and if that discount should differ depending on from which averaging set the credits come. For example, in the “2026 service class pull-ahead credits” provision in 40 CFR 1036.150(a)(4), credits generated from the certification of Heavy HDE can be used to certify Medium HDE after a 10 percent discount is applied. A credit discount can help address the concern of credit emissions value inconsistency between averaging sets. Emission credits are determined from the difference in the FEL of the engine family and the standard (in mg/hp·hr), the work over the duty cycle miles (in hp·hr/miles) for each engine family, the number of engines, and the useful life (in miles) of engines in each averaging set. There are uncertainties in real-world work and miles traveled for the range of vehicles in an averaging set compared to the certification duty cycles, and the useful life of each averaging set can have a different relationship to the real-world operating life of the range of engines in the averaging set. These uncertainties could result in an underestimate or overestimate of the true value of the emissions credit. However, since the existing averaging sets were established, the EPA has accepted the uncertainties within averaging sets such that the parameters used in the credit calculation generally represent the engines within each averaging set and ensure that a calculated emissions credit has a consistent relationship to an emissions change in the environment within a given averaging set.

However, the uncertainties are compounded when considering the relationship between calculated and real-world emissions across averaging sets, and the use of a discount factor can help balance those uncertainties to ensure that credit trading between service classes is environmentally protective.

55 FR 30585 at 30588.

The EPA notes that if credit use across averaging sets is allowed in the final rule of this proposal, this would be optional and would not require any company to make use of the flexibility. The EPA notes engine manufacturers may have concerns with the change in the competitive landscape, since finalizing such an allowance close to the implementation date could result in only benefiting manufacturers that sell engines in more than one engine service class.

Finally, the EPA requests comment on the possible economic and emissions impacts of allowing credits to be used across averaging sets in the MYs 2027 and later program. The EPA also requests comment on any reliance interest engine manufacturers or others may have, including the specific reliance concerns and available data supporting a reliance argument.

See section II.E of this preamble.

4. Combination of the Requests

The EPA recognizes that the three requests for comment in this preamble section III.D. have potential interactions. For example, if the EPA were to finalize all three elements (an increase in the NO

FEL Cap, use of pre-MY 2022 NO

credits, and use of NO

credits across engine service classes), this could reduce the compliance costs for some companies, while potentially negatively impacting other companies due to a change in the competitive landscape. The same is true if two of the three credit flexibilities were included in the final rule of this proposal, independent of the combination, as not all companies have pre-MY2022 NO

credits, and not all heavy-duty engine companies sell engines in multiple service classes. The EPA requests comment on the potential interactions of these three credit flexibilities, including the impacts on the overall emissions reductions of the MYs 2027 and later program, the costs of the program, and any potential impacts on the competitive landscape for the regulated industry. (C-11).

E. Other Regulatory Amendments

To further aid in the implementation of the MYs 2027 and later heavy-duty engine emissions program, the EPA is proposing discrete revisions across a range of issues to correct or clarify regulatory provisions. This section III.E describes the background and purpose of many of those individual proposed amendments. In several cases, this section III.E also describes the types of proposed amendments without listing all the specific instances for which the EPA is proposing to change the regulations in the same manner. For example, some proposed amendments simply correct inaccurate cross-references, fix typographical errors, reproduce figures, tables, or equations to correct publishing errors, remove obsolete or redundant content, or make modest changes to maintain consistent terminology or improve clarity. The EPA is also proposing to update the regulations to reflect office reorganization within the Agency, with new work unit titles where staff contact information is provided.

It is important to note that while this proposed rule includes a range of amendments to the regulations, the EPA is not reopening and is neither proposing nor requesting comment on the changes to underlying standards or program requirements.

1. Proposed Revisions for Heavy-Duty Engines (40 CFR part 1036)

a. 40 CFR Part 1036, Subpart B—Emission Standards and Related Requirements

Heavy-duty engine manufacturers have the option to test hybrid powertrains rather than testing an engine alone to demonstrate compliance with the emission standards of 40 CFR part 1036. In 40 CFR 1036.101(b), the EPA proposes to further clarify the existing requirement that manufacturers can only use the powertrain testing option if they demonstrate compliance with all the requirements of 40 CFR part 1036. For example, if an engine manufacturer chooses powertrain testing to demonstrate that their hybrid configuration meets the engine criteria pollutant standards of 40 CFR 1036.104, they must also use powertrain testing to demonstrate that it meets the OBD requirements of 40 CFR 1036.110.

Under existing 40 CFR 1036.104(a)(3), compression-ignition engine NO

standards for off-cycle testing are adjusted for ambient temperature over a shift day. In Table 3 to 40 CFR 1036.104(a), the EPA proposes to revise the table heading for the adjustment column from “Temperature adjustment” to a more accurate and clearer “Temperature-based adjustment for NO

”. The EPA is also proposing to revise the footnote in Table 3 to provide further clarity on the units for temperature in the adjustment equation and note the final units of g/hr for Bin 1 and mg/hp·hr for Bin 2 for the temperature-based adjustment to the specified NO

standard.

The EPA is proposing to reorganize the existing introductory text of 40 CFR 1036.125 to begin with the instruction to manufacturers to give the ultimate purchaser of each new engine written instructions for maintaining and using the engine, consistent with other parts. The EPA is also proposing corrections to Table 1 to paragraph (a)(2) of § 1036.125 to add missing parentheses, and to correct the maintenance interval for Medium HDE “Catalyst system components, EGR system components

(other than filters or coolers), particulate filtration system components, and turbochargers” in Table 2 to paragraph (a)(2) of § 1036.125 by replacing the 50,000 miles interval with 150,000 miles as intended, consistent with the interval for Heavy HDE. The EPA is proposing to revise Table 1 to paragraph (a)(2) of § 1036.125 to allow shorter minimum maintenance intervals for catalyst substrates and particular filter substrates before MY 2030, consistent with proposal to delay the longer regulatory useful life periods for heavy-duty engines. In Table 1, the EPA proposes that the minimum maintenance interval mileages for catalyst substrates and particulate filter substrates that would apply to MYs 2029 and earlier engines would match the useful life mileages that apply for those MYs. The proposed hours-based intervals are calculated using an average speed of 33 miles per hour, consistent with the existing intervals.

Under the existing maintenance regulations of 40 CFR 1036.125, owners are responsible for properly maintaining their engines, and the regulation notes in paragraph (g) that this generally includes paying for scheduled maintenance. The EPA is proposing to revise 40 CFR 1036.125(g) to further clarify that an owner's responsibility generally includes paying for scheduled maintenance even if it is within the warranty period. As part of their certification, a manufacturer must demonstrate that any critical maintenance must occur at a given interval, and the maintenance intervals for certain components (

e.g.,

spark plugs, crankcase filters) may be more frequent than the warranty period of an engine but that does not suggest a manufacturer must cover those costs for scheduled maintenance.

The EPA notes that there are circumstances in which a manufacturer offers maintenance for a component free of charge to their customers as part of the maintenance demonstration under 40 CFR 1036.125(a)(1)(iv) or because it is required under the conditions specified in 40 CFR 1036.125(g). In these cases, the maintenance instructions within the owner's manual would include this information and the owner would not be responsible for paying for scheduled maintenance on that component.

See the emission-related warranty periods specified in 40 CFR 1036.120(b) and the minimum maintenance intervals specified 40 CFR 1036.125(a).

The EPA is proposing to revise the installation instruction provisions under 40 CFR 1036.130. This proposed revision would add a new paragraph (b)(10) providing the manufacturers certifying powertrain families, under 40 CFR part 1036, would include a description of any limitations on the type or configuration of vehicles in which the powertrain could be installed in their installation instructions for vehicle manufacturers.

The EPA is proposing to add a new interim provision 40 CFR 1036.150(aa) that would allow engine manufacturers to use deterioration factors generated from durability data generated with service accumulation hours determined with prescribed test sequence 1 and 2 speeds of 40.26 mi/hr and 44.48 mi/hr respectively in 40 CFR 1036.245, for engine families that qualify for carryover certification as described in 40 CFR 1036.235(d). The EPA is proposing to eliminate the default average vehicle speeds of 40.26 mi/hr and 44.48 mi/hr for test sequences 1 and 2, respectively, in 40 CFR 1036.245(c)(7) that are used in the total service accumulation hours calculated using Equation 1036.245-1. The default values underestimate the aging hours for Light and Medium HDE because the vehicles in which these engines are installed are vehicles that operate at lower average speeds than those of test sequences 1 and 2. This can result in total service accumulation hours that are too low, which results in inadequate time to adequately account for catalyst chemical exposure and thermal exposure due to infrequent regeneration events. The current solution, to address the underestimation of aging hours from the use of average vehicle speeds of 40.26 mi/hr and 44.48 mi/hr, is for engine manufacturers to extend the service accumulation hours beyond those calculated using Equation 1036.245-1, which ensures that deterioration factors determined using the current default test sequence average vehicle speeds are not deficient. Given this current solution, the EPA is proposing to allow their continued use for carryover engine families. More details on this topic can be found in the discussion surrounding the updates the EPA is proposing to 40 CFR 1036.245 in section III.E.1.b of this preamble.

The EPA is also proposing revisions to the OBD provisions of 40 CFR 1036.110. The 2023 Final Rule included a requirement for manufacturers to identify additional data stream signals as freeze-frame conditions. The EPA is proposing to amend 40 CFR 1036.110(b)(8) to more clearly describe how this requirement interfaces with the California Air Resources Board (CARB) regulation at 13 California Code of Regulations (CCR) 1971.1(h)(4.3).

The 2023 Final Rule included a requirement for manufacturers to submit additional information as needed to demonstrate that they meet the OBD requirements in 40 CFR 1036.110 that are not covered by CARB's OBD program. The EPA is proposing to clarify in 40 CFR 1036.110(b)(11)(i) that for additional EPA OBD requirements, such as in-cab display and data stream parameter requirements, this demonstration can be achieved through attesting that these requirements are met, in lieu of submitting additional test data.

The 2023 Final Rule included a requirement at 40 CFR 1036.110(c)(1) for manufacturers to identify fault codes with an in-cab display for any SCR- or diesel particulate filter (DPF)-related condition causing the engine to initiate a speed reduction or torque derate. First, the EPA is requesting comment on whether in-cab display information requirements should remain part of the “diagnostic system.” (C-12). The required in-cab display information is not always part of the OBD system which may complicate the usual standardization efforts for diagnostic system elements. Second, the EPA has learned that some derate conditions may occur without the engine's diagnostic system generating a fault code to display. The fault code might materialize after some continued operation, or it may not come at all. To address this concern, the EPA is proposing to amend the in-cab display provision to require the displayed fault code “as applicable”. The engine's diagnostic system would need to display any code that is available, as applicable. This proposed amendment is intended only to allow manufacturers to design their in-cab display to omit a requirement to display a code that has not yet been set.

The EPA is also proposing to revise the language in 40 CFR 1036.110(b) to clarify the intended incorporation by reference in 40 CFR 1036.810 of 13 CCR 1971.1 such that it matches the Agency's position that diagnostics for hybrid components are only required if hybrid components are optionally certified to criteria pollutant standards with the engine through powertrain testing.

The EPA is proposing to remove the requirements for additional data-stream parameters for spark ignition engines in 40 CFR 1036.110(b)(10). This requirement is not useful for some applications such as natural-gas based spark ignition engines for which it would generally report a value of zero, which would be confusing to serviceability operations and not

aligned with the intent of adding data-stream parameters.

The EPA is proposing to incorporate certain changes CARB intended to make to OBD through its 2022 OBD update and its 2025 Omnibus Amendment rule.

49 50

The EPA is proposing to align OBD test procedures with EPA test procedures. Specifically, the EPA is proposing that manufacturers can use the Federal Test Procedure cycle as defined in 40 CFR 1036.512, and the Supplemental Emission Test as defined in 40 CFR 1036.510 in lieu of the procedures defined in 13 CCR 1971.1(c). Similarly, the EPA is proposing that manufacturers can use the accelerated aging test procedure and requirements in 40 CFR 1036.245 instead of the testing and data collection requirements in 13 CCR 1971.1(i)(2.3.4). This proposed change would help reduce testing burden and harmonize test procedures for OBD and emission-related testing.

California Air Resources Board. (2025) “Appendix A-1: Proposed Amendments to Omnibus Title 13 Regulation Order.”

https://ww2.arb.ca.gov/rulemaking/2025/orhdomnibus.

California Air Resources Board. (2022) “Final Regulation Order: Section 1971.1 of Title 13, CCR.”

https://ww2.arb.ca.gov/resources/documents/obd-ii-regulations-and-rulemaking.

The EPA is proposing to allow manufacturers to use a test-out threshold of 30 percent instead of the 15 percent referenced in 13 CCR 1971.1(e)(8.2.4)(A)(iii) for catalyzed PM filters to be exempt from non-methane hydrocarbon (NMHC) conversion capability monitoring.

California Air Resources Board. (2025). See page 90 of “Appendix A-1: Proposed Amendments to Omnibus Title 13 Regulation Order.”

https://ww2.arb.ca.gov/rulemaking/2025/orhdomnibus.

During the development of CARB's 2022 OBD update, CARB included relaxation of requirements for feedgas generation performance monitoring due to concerns regarding the lack of feasible monitoring strategies.

The EPA is proposing to harmonize the Agency's regulations with changes to NMHC catalyst and DPF feedgas requirements finalized in CARB's 2022 OBD update. Specifically, the EPA is proposing that for OBD systems that have an NMHC catalyst conversion efficiency monitor that fulfills the requirements of 13 CCR 1971.1(e)(5.2.2), the manufacturer may use the NMHC catalyst conversion efficiency monitor to fulfill the feedgas generation performance monitoring requirements of 13 CCR 1971.1(e)(5.2.3)(B).

California Air Resources Board. (2022). See page 21 of the “Final Regulation Order: Section 1971.1 of Title 13, CCR.”

https://ww2.arb.ca.gov/resources/documents/obd-ii-regulations-and-rulemaking.

Further, for OBD systems that have a catalyzed PM filter NMHC conversion monitor or are exempt from the catalyzed PM filter NMHC conversion monitoring requirements in accordance with section 13 CCR 1971.1(e)(8.2.4)(A), the manufacturer is not required to meet the feedgas generation performance monitoring requirements of 13 CCR 1971.1(e)(8.2.4)(B).

The EPA is also proposing the alternative approach of entirely waiving the requirements for catalyzed PM filters in 13 CCR 1971.1(e)((8.2.4)(A) and (B) based on instead meeting the PM filtering performance requirements in 13 CCR 1971.1(e)(8.2.1). These flexibilities are important to avoid fault codes being set that may improperly indicate a malfunctioned catalyst. The NMHC impacts can be so low that they are not reliably measurable and test results can instead be strongly influenced by test-to-test variation. The use of the DPF Efficiency monitor is appropriate because DPF's generally experience mechanical failure prior to catalyst failure due to their operating conditions.

California Air Resources Board. (2022). See page 35 of the “Final Regulation Order: Section 1971.1 of Title 13, CCR.”

https://ww2.arb.ca.gov/resources/documents/obd-ii-regulations-and-rulemaking.

The EPA is also proposing to clarify that the regeneration events counter associated with the in-cab display requirements in 40 CFR 1036.110(c)(2) is specific only to active and completed regeneration events. Similarly, the EPA is proposing to clarify that the requirements in 40 CFR 1036.110(c) are to display on-demand the “lifetime” rate of DEF consumption, not “historical.”

The EPA is proposing to revise the in-cab display requirements in 40 CFR 1036.110(c)(1). These existing requirements state that when SCR- or DPF-related problems occur that result in engine derates (including inducements), that manufacturers must make the fault code responsible for the derate available on-demand in the cab. With the proposal to replace SCR-related derates with visible and/or audible notifications, as described in section V of this preamble, the EPA is proposing to update the text to clarify that this requirement pertains to fault codes that change the status of an aftertreatment system and isolate the display requirements that apply broadly from those that apply to AECDs that result in an engine derate (

i.e.,

for any DPF-related or SCR-related derates that may remain). The EPA notes that some AECDs may continue to result in engine performance derates for engine protection events such as low coolant to avoid overheating and damaging the engine or catastrophic damage to the aftertreatment system. The intent of the original requirement in 40 CFR 1036.110(c)(1) was to properly communicate what problem caused an engine derate to facilitate quick repair. The proposed amendment is relevant even if the EPA finalizes the proposal in section V.C.2 to replace SCR-related derates with visible and/or audible notifications because the display requirements also apply and would continue to apply to derates caused by the SCR or DPF system.

Finally, the EPA is proposing to clarify in 40 CFR 1036.110(c)(4) that manufacturers can use discretion to decide what to safely display while the vehicle is in-motion, which may be a more limited set of the required information than is displayed when the vehicle is stationary.

b. 40 CFR Part 1036, Subpart C—Certifying Engine Families

The EPA is proposing to amend 40 CFR 1036.205(r)(1) by referring to “physically adjustable parameters” instead of “practically adjustable parameters” to conform to the terminology established in the existing, more detailed provisions for adjustable parameters in 40 CFR 1068.50. Under those existing provisions, the regulations describe how to determine the practically adjustable range for both physically adjustable operating parameters and programmable operating parameters.

The EPA is proposing to clarify in 40 CFR 1036.235(b) how manufacturers test their flexible-fuel emission-data engines. Specifically, manufacturers may operate with the fuel mixture that best represents in-use operation or is most likely to have the highest NO

emissions, or they may test with both fuels separately if they demonstrate to the EPA that intermediate mixtures are not likely to occur. The EPA is also proposing to replace the incorrect paragraph (a) reference in 40 CFR 1036.235(e) with a reference to paragraph (b) that describes testing.

The EPA is proposing a revision in 40 CFR 1036.240(b) that adds “duty-cycle” to clarify the applicable emission standard. The EPA also proposes to add a provision to 40 CFR 1036.240(c) allowing for assigned deterioration factors for small-volume engine families. Each manufacturer would be able to use assigned deterioration factors for any number of engine families, up to a combined annual production volume of 10,000 engines. This would restore certification provisions that apply through MY 2026 under 86.094-14. As

part of the migration to 40 CFR part 1036, the EPA set up deterioration factor requirements in 40 CFR 1036.240 and 1036.245 to broadly allow companies to share deterioration factors across families using similar technologies. Manufacturers have raised concerns that this approach does not work for engines using alternative fuels or other technologies that are not uniformly deployed across their product line. Allowing assigned deterioration factors for small-volume engine families would remove the significant test burden associated with durability testing for those niche products. Such engines remain subject to the emission standards throughout the useful life.

The EPA is proposing changes throughout 40 CFR 1036.245 to accommodate the proposed addition of an accelerated aging procedure for heavy-duty spark-ignition engines in 40 CFR part 1065, subpart L. See the discussion of these changes in section III.E.3.j of this preamble.

The EPA is proposing to revise 40 CFR 1036.245(c) to clarify that the deterioration factors must be based on representative deterioration due to infrequent regeneration and that engine dynamometer hours accumulated during emission testing do not count toward total service accumulation hours. The EPA is proposing to revise 40 CFR 1036.245(c)(1) to clarify that the minimum engine dynamometer service accumulation hours include the dynamometer engine stabilization time accumulated prior to performing the low-hour emission measurement. The EPA is proposing to revise 40 CFR 1036.245(c)(2) to allow manufacturers to use the average of multiple emission tests to determine emission values used to calculate the deterioration factor. The EPA is proposing to allow this to minimize the influence of any variability associated with the emission measurement.

In 40 CFR 1036.245(c)(4), (c)(4)(i), and (c)(4)(ii) the EPA is proposing to clarify that, for hybrid powertrains, manufacturers may conduct service accumulation under paragraphs (c)(4)(i) or (ii) without the hybrid components by following 40 CFR 1037.551. The EPA is also proposing to add new paragraphs (c)(4)(i)(F) and (ii)(F) to clarify how to perform cycle validation during service accumulation, and state that cycle validation needs to be performed on at least one of each duty cycle type each time the engine is operated over the duty cycle sequence. These proposed clarifications are appropriate for the same reason described in the discussion of the proposed addition of the new 40 CFR 1036.245(c)(5) and (6).

The EPA is also proposing to clarify that spark-ignition engines may exclude the LLC from the service accumulation test sequences in 40 CFR 1036.245(c)(4)(i) and (ii) as these engines are not subject to certification over the LLC. The LLC has normalized speeds up to 106.4 percent. For spark-ignition engines with high-speed governors that are just above the maximum power of the engine, a normalized speed of 106.4 percent puts the test speed above the governed speed when using typical denormalization procedures. Thus, requiring service accumulation over the LLC for spark-ignition engines would require the manufacturer to apply the alternate maximum test speed provision in 40 CFR 1065.610(a)(2) not only for the LLC, but also the Federal test procedure (FTP) and supplemental emissions test (SET) duty cycles. Rather than force the engine manufacturers to test the engine over the FTP and SET duty cycles at an alternate maximum test speed just to accommodate the LLC, a duty cycle to which they are not required to certify, the EPA is proposing to update 40 CFR 1036.245(a)(3) to allow exclusion of the LLC for spark-ignition engines for the two-engine service accumulation test sequences.

The EPA is proposing to add a new 40 CFR 1036.245(c)(5) and (6) that includes alternative duty cycle validation criteria for service accumulation for engine dynamometer testing. Meeting the existing cycle validation criteria that are required for engine certification testing is not as important for service accumulation when using an engine dynamometer, especially when most of the service accumulation on the aftertreatment system is being done via the accelerated aging procedure in 40 CFR part 1065, subpart L. Furthermore, if one of the many duty cycles run during service accumulation failed the existing cycle validation criteria, the EPA recognizes that it would be impractical to void all of the service accumulation testing to date because one duty cycle did not pass the cycle validation criteria. The proposed revised cycle validation criteria would help ensure that the duty cycles are followed appropriately for service accumulation when using an engine dynamometer, while also avoiding situations in which a duty cycle is deemed invalid. Under the proposed provisions, setting the dynamometer command to minimum and using the operator demand to control speed for nonmotoring dynamometers would not adversely affect the aging of the aftertreatment during service accumulation and would allow service accumulation to be carried out on an eddy current dynamometer (which cannot motor due to load imparted on the engine from cooling water and bearing drag). Allowing the use of eddy current dynamometers under the proposed provisions would also open up additional test cells for use in service accumulation testing and increase test facility throughput.

In 40 CFR 1036.245(c)(7), the EPA is proposing to clarify that emission measurements for all regulated pollutants must be taken after the manufacturer finishes the required service accumulation that is conducted on an engine dynamometer.

The EPA is proposing to add clarification to 40 CFR 1036.245(c)(8)(i) that the useful life milage used in the calculation of the total service accumulation, representing hours of engine operation over the useful life, is based on the longest useful life for which the deterioration factors will be used. The EPA is proposing to eliminate the default average vehicle speeds of 40.26 mi/hr and 44.48 mi/hr for test sequences 1 and 2 in 40 CFR 1036.245(c)(7) that are used in the total service accumulation hours calculated using Equation 1036.245-1. The default values underestimate the aging hours for Light and Medium HDE because the vehicles in which these engines are installed are vehicles that operate at lower average speeds than those of test sequences 1 and 2. This can result in total service accumulation hours that are too low, which results in inadequate time to accurately account for catalyst chemical exposure and thermal exposure due to infrequent regeneration events. The current solution, to address the underestimation of aging hours from the use of average vehicle speeds of 40.26 mi/hr and 44.48 mi/hr, is for engine manufacturers to extend the service accumulation hours beyond those calculated using Equation 1036.245-1, which ensures that deterioration factors determined using the current default test sequence average vehicle speeds are not deficient. Under the proposed changes, manufacturers are now directed to develop their own average vehicle speed based on the applications used for determining the thermal exposure for the engine family as described in the field data discussion in 40 CFR 1065.1131 or 40 CFR 1065.1147. The EPA is also proposing to require the use of an average vehicle speed of 20 mi/hr if the average speed from the field data is less than 20 mi/hr. This is because a speed of lower than 20 mi/hr will result in the service accumulation hours exceeding the

useful life hours in Table 4 to paragraph (e) of 40 CFR 1036.104. The EPA proposes to allow the use of previously developed deterioration factors that used the test sequence default average vehicle speeds for carryover engine families in 40 CFR 1036.150(aa) for engine families that qualify for carryover certification as described in 40 CFR 1036.235(d). The EPA is also proposing in 40 CFR 1036.245(c)(8)(ii) to clarify that the service accumulation duration could be different if the testing is used for multiple primary intended service classes.

The EPA is proposing to clarify in 40 CFR 1036.245(c)(9) how to calculate the number of hours of testing remaining to finish service accumulation over the useful life for the accelerated bench aging of aftertreatment devices portion of the testing.

The EPA is proposing to clarify in 40 CFR 1036.245(c)(10) on what emission data engine the aftertreatment devices can be reinstalled once the accelerated aging portion of the testing has been completed.

The EPA is proposing to clarify in 40 CFR 1036.245(c)(11) and (12) references to applicable sections in the procedure in this section to enhance the ability to follow what is being required.

The EPA is proposing to revise 40 CFR 1036.245(c)(13) to remove the sentence instructing the manufacturer to create a linear curve fit of the emission measurements if the deterioration factor testing included multiple test points and proposing that the deterioration factor is determined using the emission data from the low-hour and useful life test points. Since the emissions measurements are made at the low-hour and useful life for each standard, a linear curve fit of the emission data is not needed and could result in unrepresentative results.

The EPA is proposing to add a new 40 CFR 1036.245(c)(14) to clarify how to determine separate deterioration factors for engines corresponding to a shorter useful life.

The EPA is proposing to correct a typographical error regarding the spelling of “stabilized” in 40 CFR 1036.246(b)(1)(i)(C).

c. 40 CFR Part 1036, Subpart E—In-Use Testing

The EPA is proposing to revise 40 CFR 1036.410(c) to correct a paragraph reference error. The section currently requires the manufacturer to notify the EPA before disqualifying any vehicle based on an illuminated malfunction indicator light (MIL) or stored OBD trouble codes as described in 40 CFR 1036.415(b)(2). However, 40 CFR 1036.415(b)(2) concerns the appearance of a MIL during in-use testing and how to address the MIL. The correct paragraph reference should be 40 CFR 1036.415(b)(3), which addresses vehicle disqualification due to the presence of an MIL if the vehicle cannot be repaired in a timely manner, which is the only provision that allows disqualification of a vehicle due to an illuminated MIL.

The EPA is proposing to revise 40 CFR 1036.415(d), 1036.420(c) and (d), and 1036.430(a)(3)(vii) to replace the term “windows” with “test intervals” as the Agency believes this term is clearer given that the 300 seconds of data that are averaged for binning purposes constitute a test interval. The EPA is also proposing to clarify that if a manufacturer does not measure total hydrocarbon emissions (THC) during the off-cycle emission test, they would set

THCmeas

to zero in the chemical balance calculations required under 40 CFR 1065.655.

The EPA is proposing to add a new 40 CFR 1036.415(h) to clarify that, for hybrid powertrains that do not operate the engine at zero vehicle speed, the 1-Hertz (Hz) emission rate for all pollutants after the engine has started must be set to zero when the engine is off, the vehicle speed is zero, and the powertrain is keyed on and that the data points should not be excluded under 40 CFR 1036.530(c)(3)(ii). This proposed provision is similar to what currently exists in 40 CFR 1036.415(g) for stop-start and automatic engine shutdown systems and allows test intervals to be accrued and counted toward the idle bin minimum test interval limits when the engine is off.

The EPA is proposing to provide clarifications in 40 CFR 1036.420(d) on extending testing over multiple shift days to combine test intervals to achieve the minimum test interval requirement for each bin. The EPA proposes to include four examples of what circumstances might lead to the extension of testing to an additional shift day: data lost from gas analyzer range validation failure (40 CFR 1065.935(g)(5)(i)), drift validation failure (40 CFR 1065.935(g)(5)(ii)), excluded data (40 CFR 1036.530(c)(3)), and insufficient operation during the shift day (40 CFR 1036.415(f)).

The EPA is proposing changes to 40 CFR 1036.420(d) and 40 CFR 1036.425(c) to clarify how to determine the average ambient temperature used to calculate the off-cycle NO

emission standard in 40 CFR 1036.104(a)(3). The EPA is proposing in 40 CFR 1036.420(d) to determine the mean ambient temperature,

amb

, by averaging the continuous ambient temperature,

amb

, over all shift days needed to reach the minimum test interval requirements for each bin. The EPA is proposing changes to 40 CFR 1036.425(c) to clarify that the mean ambient temperature is determined by taking the mean of

amb

from each of the engine tests.

The EPA is proposing in 40 CFR 1036.420(d)(2) to clarify that, for hybrid powertrains that do not operate the engine at zero vehicle speed, the manufacturer may populate Bin 1 with additional test intervals by setting the 1-Hz emission rate for all regulated pollutants to zero as described in 40 CFR 1036.415(h). This proposed provision is similar to what currently exists in 40 CFR 1036.420(d)(2) for stop-start and automatic engine shutdown systems and allows Bin 1 to accumulate exactly 2,400 Bin 1 test intervals in instances in which the number of test intervals falls short.

The regulation at 40 CFR 1036.430(a)(1)(ii) instructs manufacturers to include a summary of the vehicles they have disqualified from in-use testing and the reasons they disqualified them. The EPA is proposing to revise 40 CFR 1036.430(a)(1)(ii) to clarify that for vehicles disqualified from in-use testing for a MIL, the test report should include a description of the illuminated MIL or stored OBD trouble codes. The intent in 40 CFR 1036, subpart E, was for manufacturers to notify the EPA of any vehicles disqualified from in-use testing due to MIL illumination or OBD fault codes, and this proposed addition to 40 CFR 1036.430(a)(1)(ii) clarifies what information is expected to support vehicle disqualification actions.

d. 40 CFR Part 1036, Subpart F—Test Procedures

The EPA is proposing to revise 40 CFR 1036.501(h) to clarify that, for nonhybrid engines that qualify and use the powertrain test procedures, the provisions specified for nonhybrid powertrain testing should be used. For example, in 40 CFR 1036.520, for nonhybrid powertrains, continuous rated power (

contrated

) equals rated power (

rated

The EPA is proposing to add a new Figure 1 to 40 CFR 1036.501 to provide an example of a test sequence for determining criteria pollutant emissions for plug-in hybrid powertrains.

The EPA is proposing to revise 40 CFR 1036.510(c) to clarify that testing over the SET duty cycle is required for both compression-ignition and spark-ignition engines as SET standards exist for both combustion types in 40 CFR 1036.104.

The EPA is proposing to revise Figure 1 of 40 CFR 1036.510, Figure 1 of 40 CFR 1036.512, and Figure 2 of 40 CFR 1036.545 by removing “Engine Start” to eliminate confusion for plug-in hybrid powertrains in which the engine starts multiple times during charge-depleting operation to blend engine and electric power.

The EPA is proposing to revise 40 CFR 1036.510(g), 1036.512(c), 1036.514(d), 1036.530(j)(1), and 1036.545(o)(7) to clarify that, for hybrid powertrains, when determining total work over a test interval in 40 CFR 1065.650(d), system power (

sys

) from 40 CFR 1036.520(f) is used in place of the shaft power determination in 40 CFR 1065.650(d)(2) and paragraph 40 CFR 1065.650(d)(6) does not apply. In 40 CFR 1036.514(d), the EPA is also proposing to clarify that manufacturers would not exclude accessory loads when calculating total work over the LLC. The proposed clarification would be helpful because the direction in 40 CFR 1065.650(d)(6) to “set all power values to zero during idle periods with a corresponding reference torque of 0 N·m” could potentially lead someone to incorrectly believe that the accessory loads in 40 CFR 1036.514(c) should be excluded from the calculation of total work.

The EPA is proposing to revise 40 CFR 1036.510(b)(2) to clarify that, for hybrid powertrain testing, the manufacturer may set the battery to the lowest state-of-charge level that results in charge sustaining operation after preconditioning to help meet the net-energy-change requirement in 40 CFR 1036.545(a)(6). The EPA is also proposing to add a reminder to follow 40 CFR 1065.610(d)(3)(ii) to apply optional declared accessory loads. The EPA is proposing to revise 40 CFR 1036.510(b)(2)(iii) to correct the solution to the example problem for the calculation of

The EPA is proposing to republish the SET duty cycle in 40 CFR 1036.510, Table 1, as the current version has only three significant figures for the road-grade coefficients. This table was intended to originally be published with four significant figures for these coefficients to be consistent with the road grade coefficients in the transient test interval for spark-ignition engines and powertrains under 40 CFR 1036.512 in paragraph (b) of Appendix B to 40 CFR part 1036, the transient test interval for compression-ignition engines and powertrains under 40 CFR 1036.512 in paragraph (c) of Appendix B to 40 CFR part 1036, and the transient duty cycle for compression-ignition engines and powertrains under 40 CFR 1036.514 in paragraph (d) of Appendix B to 40 CFR part 1036. The number of significant figures influences the amount of work the engine does over the duty cycle. Updating the SET road-grade coefficients from three to four significant figures would result in a relatively large difference in work (up to 90 percent) for a few of the low power mode points, but would only result in a total cycle work difference of less than 0.2 percent.

The EPA is proposing to remove the charge-depleting (CD) test requirement for criterial pollutant testing for SET in 40 CFR 1036.510(d). The criteria pollutant testing requirements for CD testing are currently not consistent with how CD testing has been historically carried out on light-duty vehicles, for which CD testing is only required on the FTP for criteria pollutant testing. When the EPA originally drafted and finalized the CD criteria pollutant testing requirements in the 2023 Final Rule, the Agency intended to put heavy duty testing on par with light duty testing for plug-in hybrids. For that to occur, CD testing for heavy-duty engines should only apply to the FTP. Note that this proposed change requires moving the CD test procedure from 40 CFR 1036.510(d)(2) through (4) to 40 CFR 1036.510(e)(1) through (3) as both charge-sustaining (CS) and CD testing are still required over the SET for the National Highway Transportation Safety Administration's (NHTSA) fuel economy program.

The EPA is proposing to update 40 CFR 1036.510(g) by adding new subparagraphs (1), (2), and (3) to provide three exceptions when calculating work over the test interval from hybrid powertrains as described in 40 CFR 1065.650(d). These exceptions require determining shaft power at each point in the test interval using the system power from 40 CFR 1036.520(f) instead of calculating it according to 40 CFR 1065.650(d)(2) and not setting power values to zero as described in 40 CFR 1065.650(d)(6). Under this proposal, the first exemption is to keep powertrain testing consistent with engine testing by requiring the shaft power to be determined at the engine shaft regardless of where power is measured. Under this proposal, the second exemption is needed because the engine in a hybrid powertrain may start multiple times during the test and it would be unrepresentative to zero out this power, since the energy to start the engine is coming from the powertrain. Under this proposal, the third exemption is needed to correct the total work for criteria pollutant testing by the additional energy from the battery if the net energy change (NEC) over the duty cycle is greater than 1 percent and less than or equal to 5 percent. The correction is needed because the total work of the powertrain includes both the energy produced by the engine and the energy that comes from the battery. The EPA is not proposing to allow correction to total work if the NEC is less than −1 percent since the additional energy in the battery may have come from regenerative braking near the end of the duty cycle. In addition, the EPA is proposing in 40 CFR 1036.545(a)(6) to increase the tolerance of NEC from ± 1 percent to ± 5 percent.

The EPA is proposing to update 40 CFR 1036.512(c) to clarify the total mass of each constituent and the total work over the test interval is determined as described in 40 CFR 1036.510(g). This would make this process consistent across duty cycles and test intervals.

The EPA is proposing to update 40 CFR 1036.512(d) to clarify the determination of criteria pollutant emissions over the FTP for plug-in hybrid powertrains, including adding a new 40 CFR 1036.512(d)(1)(i) to carry out preconditioning of the engine or powertrain as described in 40 CFR 1065.518(c)(1).

The EPA is proposing to update the composite emission calculation for criteria pollutant determinations for CD testing of plug-in hybrid powertrains in 40 CFR 1036.512(d)(1)(iv). The FTP duty cycle was designed such that the engine would have 20 minutes of operation, starting from a cold soak, resulting in fully warmed-up operation prior to the soak period that precedes the start of the hot-start test interval. Testing of plug-in hybrid vehicles could result in a scenario in which, during the CD testing, the engine starts near the end of the test interval, resulting in the engine not being fully warmed up by the end of the test interval. This would result in the inclusion of what would be considered some cold-start operation in the hot-start test interval, which could have the potential to bias the composite emission value high, as the emissions from the hot-start test interval are 85.7 percent of the composite test cycle. To ensure that FTP emissions from plug-in hybrid powertrains are treated in the same manner as regular or hybrid powertrains, the EPA is proposing to calculate the composite emission value as follows:

1. Determine the average of the sum of emissions over the first test intervals in which engine operation occurs and the subsequent test interval and add that to six times the emissions from the third

test interval, which is the true hot-start test interval.

2. Then divide that value by the average of the sum of the work over the first test intervals in which engine operation occurs and the subsequent test interval plus six times the work from the third test interval, which is the true hot-start test interval.

The EPA is proposing to update Figure 1 to paragraph (d)(4) of 40 CFR 1036.512 to remove the arrow that denotes “engine start” as the engine could start in other FTP test intervals without the engine entering charge sustaining operation. The EPA is also proposing to update the figure to support the proposed changes to 40 CFR 1036.512(d)(1)(iv).

The EPA is proposing to add a new Figure 1 to paragraph (d)(5) of 40 CFR 1036.512 to provide an example of the FTP CS criteria pollutant test sequence to provide an illustration of how the test sequence is carried out.

The EPA is proposing to revise 40 CFR 1036.514(a)(1)(i) to clarify what is meant by the statement that “Declared idle torque must be zero.” This proposed clarification is that the use of the optional declared idle torque in 40 CFR 1065.510(f)(5)(iii) is not allowed and must be zero. The EPA is proposing to revise 40 CFR 1036.514(b) to delete the existing paragraph 40 CFR 1036.514(b)(4) because there are no adjustments made to the procedures in the referenced 40 CFR 1036.510(d). The EPA is proposing to relocate the exception to replace “SET” with “LLC” to 40 CFR 1036.514(b)(1) and renumber subsequent paragraphs as the replacement of SET with LLC should be first in the order of exceptions.

The EPA is proposing to revise 40 CFR 1036.514(d) to reference 40 CFR 1036.510(g) to calculate the total mass of each constituent and the total work over the test interval rather than describing such in this paragraph. Doing this would provide one common location for total mass and work determination, including exceptions for hybrid powertrain testing. The EPA is also proposing to add a reminder that the work for accessory loads is not excluded from the work calculation as the LLC duty cycle has accessory load requirements that must be included.

The EPA is proposing to revise 40 CFR 1036.520(d) to clarify that when the manufacturer carries out the powertrain power and vehicle speed determination, the state-of-charge of the rechargeable energy storage system (RESS) must be at a representative level to deliver maximum power. The EPA is also proposing to change the warmup cycle to the FTP, rather than a recommendation of any vehicle speed and road grade that results in 75 percent of maximum power, to standardize the warmup and provide a level playing field across engine manufacturers. The EPA is also proposing to provide an initial ramp up of the vehicle speed to five mi/hr with a five second hold at this speed as the initial acceleration was not intended to be wide-open throttle from the initial depression of the pedal, which could cause problems for some powertrain architectures. The EPA is also proposing to alter the wide-open throttle maneuver after the five second hold at five mi/hr to ramp to the maximum driver demand for full load acceleration within three seconds. The EPA anticipates that these adjustments would provide some relief to the powertrain from the aggressive acceleration.

The EPA is proposing to revise 40 CFR 1036.520(e) and (f) to replace occurrences of “transmission input” with “engine's primary output” since the output shaft of the engine is the more commonly used term to identify where system power is determined for a conventional powertrain configuration.

The EPA is proposing to revise 40 CFR 1036.520(h) and (k)(2) to change the maximum coefficient of variation (COV) of the vehicle system power from two percent to one half of one percent. The COV is used to select data that determine measured rated power, which is then used to determine rated power and continuous rated power. The EPA is proposing this change because recent data from engine hybrid testing in which the transmission is simulated have shown that measured rated power is elevated during transmission upshift events, which can result in unrepresentatively high determinations of rated power and continuous rated power. Reducing the maximum COV of measured rated power from two percent to one half of one percent would lead to more representative values of rated power and continuous rated power by reducing the influence of shifting.

The EPA is proposing to limit the values of

contrated

and

rated

, determined in 40 CFR 1036.520, for hybrids and plug-in hybrid powertrains. These values are used for determining the vehicle test mass and road grade in 40 CFR 1036.510, 1036.512, and 1036.514, using the equations in 40 CFR 1036.510. The equations were developed based on the rated power of non-hybrid diesel engines and with the assumption that hybrid powertrains would be designed to have comparable rated power to their non-hybrid diesel engine counterparts.

However, if a hybrid powertrain is designed to have higher rated power than its non-hybrid diesel engine counterparts, calculated values for vehicle test mass and road grade can result in a duty cycle that is not representative for the hybrid powertrain. To illustrate this, for example, the EPA has estimated that the Tesla Semi

has a

contrated

of 740 kilowatts (kW), which would result in a vehicle test mass of 86,630 kg (190,987 pounds) using Eq. 1036.510-1.

In this case, the vehicle test mass is not representative since the vehicle has a gross combined vehicle weight rating of 82,000 pounds. The EPA proposes to address this in 40 CFR 1036.520 by limiting the values of

contrated

and

rated

to the maximum value of rated power of non-hybrid diesel engines in that category. The EPA proposes to use the certification data for MY 2021 to 2024 diesel engines, which would result in the maximum rated power for Light HDE, Medium HDE, and Heavy HDE of 260, 340, and 450 kW, respectively.

For spark-ignition HDE, the EPA proposes using the same 260 kW value used for Light HDE. These rated power values would result in a maximum vehicle test mass of 22,009, 31,276, and 45,153 kg for Light HDE, Medium HDE, and Heavy HDE, respectively, using Eq. 1036.510-1. These values for vehicle test mass are comparatively high for vehicles with Light HDE, Medium HDE, and Heavy HDE, but the EPA's analysis supports that the power demand from the cycles would be representative when accounting for the corresponding road grade in each of the FTP, SET, and LLC duty cycles.

Memorandum to docket EPA-HQ-OAR-2026-0728. “Summary of the Development of the Powertrain FTP, SET, and LLC Duty Cycles.” June 2026.

There are currently no publicly available data on the rated power from heavy-duty plug-in hybrids. For this illustration, the EPA instead used the rated power of heavy-duty battery electric vehicles since the same e-motors can be used for plug-in hybrids and battery electric vehicles.

The

contrated

of the Tesla Semi was estimated by using the performance specifications from Tesla's website (see

www.tesla.com/semi

) of zero to 60 miles per hour (MPH) in 20 seconds, with a gross combined vehicle weight rating of 82,000 lbs and physics-based calculations for heavy-duty vehicles.

The certification data for diesel engines were used since data from diesel engines were used to develop the equations in 40 CFR 1036.510 for defining the vehicle model. Values were determined using rated power expressed to the nearest 10 kW. See Heavy-Duty Highway Gasoline and Diesel Certification Data (Model Years: 2015-Present).

www.epa.gov/compliance-and-fuel-economy-data/annual-certification-data-vehicles-engines-and-equipment.

Memorandum to docket EPA-HQ-OAR-2026-0728. “Summary of the Development of the Powertrain FTP, SET, and LLC Duty Cycles.” June 2026.

The EPA is proposing to update the emission sampling duration in the clean idle test in 40 CFR 1036.525(b)(3) from 1,200 second to 1,800 seconds to align this test with the CARB test procedure in 13 CCR 1956.8.

The EPA is proposing to add a new paragraph (c) to 40 CFR 1036.530 to add the already-existing requirement to perform drift verification as described in 40 CFR 1065.935(g)(5)(ii) and (iii). The EPA is also proposing to clarify that, for drift verification of CO and HC under 40 CFR 1065.935(g)(5)(ii), the manufacturer must determine a corrected value of the off-cycle emission values for Bin 2 in g/kW·hr using equation 1036.530-3 for each interval between analyzer verifications. Under this proposal, the corrected value is then compared to the uncorrected value or to the Bin 2 emission standard in 40 CFR 1036.104(a)(3), whichever is greater, to determine whether the drift is within the specified percentage limit in 40 CFR 1065.935(g)(5)(ii). The EPA notes that carbon dioxide (CO

) drift correction and verification is based on the mass of CO

as described in 1065.935(g)(5)(ii), which references 40 CFR 1065.550(b)(3)(ii)(A). Since there is no off-cycle standard for CO

, 40 CFR 1065.550(b)(3)(ii)(A) indicates that the verification is done on a mass basis. The EPA notes that the NO

drift verification is based on an analyzer zero-verification limit of ±2.5 parts per million (ppm) for each interval between analyzer verifications, a zero-drift limit over the shift day of ±10 ppm, and a span-drift limit for each interval between analyzer span verifications of ±4 percent of the measured span gas value. The EPA is proposing to revise 40 CFR 1036.530(c)(2) to replace the term “windows” with “test intervals” as the Agency believes this term is clearer given that the 300 seconds of data that are averaged for binning purposes constitute a test interval. The EPA is proposing to revise 40 CFR 1036.530(c)(3)(ii) to clarify that no data is valid when the operator has the engine off beyond the existing exceptions for stop-start and automatic engine shutdown systems and a proposed additional exception for certain hybrid powertrains. Hybrid powertrains for which the engine does not operate at zero vehicle speed when the powertrain is keyed on work in a similar manner to stop-start and automatic engine shutdown technologies by eliminating engine idle when the engine is off, the vehicle speed is zero, and the powertrain is keyed on. The EPA is proposing to add a cross-reference to the newly proposed 40 CFR 1036.415(h) to consistently allow this hybrid powertrain type of idle reduction technology to report an emission rate of zero during these scenarios. The EPA is also proposing to add two test interval data condition exclusions at 40 CFR 1036.530(c)(3)(viii) and (ix), gas analyzer range validation failure according to 40 CFR 1065.935(g)(5)(i) and any data that does not meet the drift criteria in 40 CFR 1065.935(g)(5)(ii). This data is currently required to be excluded under existing 40 CFR 1065.935(g)(5); however, the EPA is proposing conforming additions under this provision for completeness and clarity, to make it easier for manufacturers to locate all the exclusion requirements in one place. The EPA is additionally proposing to revise the variable description for

CO2FTPFCL

in 40 CFR 1036.530(e) to clarify that it is also the CS CO

emissions over the FTP duty cycle for plug-in hybrid powertrains. The variable

CO2FTPFCL

is used as a surrogate for work and, if the CD CO

under the existing variable description was used, it would give a reduced work value as the CO

emissions under CD operation do not represent the amount of work being done due to the hybrid contribution. The EPA is thus proposing that the CS CO

emissions must be used to properly determine the normalized CO

emission mass over a 300-second test interval for off-cycle emissions. The EPA is also proposing to revise the variable description for

CO2

in 40 CFR 1036.530(g)(1) to clarify that it is the total drift-corrected CO

emission mass over the test interval. Drift correction is required for all gaseous pollutants measured during off-cycle testing as described in 40 CFR 1065.935(g)(5)(ii). This clarification is a reminder to the manufacturer that the CO

emission mass over the test interval must be drift corrected.

The EPA is considering a revision to 40 CFR 1036.530(c)(3)(iii) to allow the existing exclusion of data during an infrequent regeneration event to include any additional time needed for emissions levels to return to normal baseline levels. Under 40 CFR 1065.680, as amended in the 2023 Final Rule, the EPA specified that the duration of a regeneration event continues until aftertreatment performance and emission levels have fully returned to normal. The EPA adopted this change for certification because the FTP, SET, and LLC duty cycles are relatively short and data from operation immediately following regeneration may unrepresentatively bias measured emissions high on the short certification cycles. In addition, 40 CFR 1065.680 is structured to capture the emissions over the complete cycle, including operation during regeneration and the period until the next regeneration event occurs. As a result, defining the operation after a regeneration event when emissions are still stabilizing as part of the regeneration does not change the calculated infrequent regeneration adjustment factor and mitigated concerns that post-regeneration operation would otherwise be treated as normal baseline operation. The EPA requests comment on whether a corresponding redefinition of the regeneration event is warranted for off-cycle testing under 40 CFR 1036.530(c)(3)(iii). (C-13). The EPA notes that operation subject to off-cycle test procedures differs from certification duty cycles in that off-cycle testing spans significantly longer durations, reducing the likelihood that post-regeneration stabilization periods would disproportionately bias emissions high. In fact, exclusion of this data may result in biasing the emissions subject to the off-cycle standards low.

40 CFR 1036.420(c) requires that Bin 1 include at least 2,400 test intervals (which is approximately 45 minutes of engine operation) and Bin 2 include at least 10,000 test intervals (which is approximately three hours of engine operation).

The EPA is proposing amendments to 40 CFR 1036.545 to clarify and improve the powertrain test procedures. The EPA is proposing to update 40 CFR 1036.545(a)(3)(i) to clarify how the transmission state (in gear or idle) should be updated in the Greenhouse Gas Emissions Hardware-in-the-Loop (GEM HIL) model. The update should occur in the driver interface block. The EPA is proposing to revise 40 CFR 1036.545(a)(3)(ii) to clarify that a human driver can be used instead of a driver model. The EPA is proposing to revise Figure 1 to paragraph (a)(11) of 40 CFR 1036.545 by removing the appearance of “drive idle” and “parked idle” duty cycles from box 7, as they are only required to be run for engines installed in vocational vehicles under the existing 40 CFR 1036.535(c). Also in Figure 1, the EPA is proposing to modify the duty cycle exceptions notes for SET, FTP, and LLC to include notes, in addition to the existing note to disregard duty cycles in step 7, to disregard the duty cycles in step 8 and to disregard steps 10 and 11 in their entirety. The EPA is proposing these additions in this figure as these steps are not relevant to testing powertrains over SET, FTP, and LLC duty cycles. The EPA is also proposing to redesignate the paragraph that the figure resides in as 40 CFR 1036.545(a)(13).

The EPA is proposing to revise Equation 1036.545-4 in the vehicle model calculations of 40 CFR 1036.545(f)(3) to clarify that cumulative distance,

-1

, used to determine the percent grade (

-1

), is calculated at each time step

to determine the cumulative distance driven from the test cycle grade profile in 40 CFR part 1037, appendix D.

The EPA is proposing to revise 40 CFR 1036.545(j)(4) to set the dynamometer target torque to zero when ramping to the target speed at 1 mi/hr/s. The proposed change is intended to minimize the energy used from the plug-in hybrid battery before the CD test sequence has started and to clarify the ramp rate. The EPA is proposing to revise 40 CFR 1036.545(j)(5) to clarify that, for plug-in hybrids, for a CD cruise cycle test sequence, the manufacturer must transition to the next duty cycle without altering the powertrain speed. This is because plug-in hybrid powertrains run the same vehicle configuration for the cruise cycles back-to-back to go from CD to CS, therefore they should not transition to zero vehicle speed between each cruise cycle. In 40 CFR 1036.545(j)(6), the EPA proposes to remove hyphens in the term “duty cycle” and clarify that the start of the next duty cycle must be within 60 to 180 seconds after shutting off the powertrain for non-plug-in hybrid and conventional powertrains. This is a conforming proposed revision as the plug-in hybrid would be able to run the duty cycles immediately per the update the EPA is proposing to 40 CFR 1036.545(j)(5).

The EPA is proposing to update 40 CFR 1036.545(m) to clarify that the manufacturer may delete points before performing the cycle validation when a hybrid engine is at idle. The EPA is also proposing to clarify that when manufacturers validate the measured output speed, they may time-align the reference and measured output speed values by up to 0.5 seconds as described in 40 CFR 1065.514(c).

The EPA is proposing to revise Equation 1036.545-9 used to create the GEM input for the simulated vehicle configurations in 40 CFR 1036.545(o)(3) to clarify that cumulative distance,

-1

, is calculated at each test interval to determine the cumulative distance driven under CD conditions, which is then used to determine the utility factor.

The EPA is proposing to revise 40 CFR 1036.545(o)(4)(i) to correct an error in the equation referenced for testing with the torque measurement at the wheel hubs. The reference should be to Eq. 1036.545-10 instead of Eq. 1036.545-8.

The EPA is proposing to revise 40 CFR 1036.555(a) to replace the term “windows” with “test intervals” as the Agency believes this term is clearer given that the 300 seconds of data that are averaged for binning purposes constitute a test interval.

e. 40 CFR Part 1036, Subpart I—Definitions and Other Reference Information

The EPA proposes several revisions to the definitions of 40 CFR 1036.801. The EPA proposes new definitions for the terms “certified configuration” and “useful life” that are used throughout 40 CFR part 1036 but not explicitly defined. The EPA proposes corrections to the definition of “designated compliance officer” to reflect a recent reorganization in the Agency's Office of Transportation and Air Quality.

The EPA is proposing to update ASTM D975, ASTM D4809, ASTM D4814, and ASTM D7467, that are incorporated by reference in 40 CFR 1036.810(a), to the latest versions. The EPA is also proposing to correct existing 40 CFR 1036.810(d)(4) by adding the missing document title for CARB's 2019 regulation “13 CCR 1971.5” in the incorporation by reference entry.

f. 40 CFR Part 1036, Appendix B—Transient Duty Cycles

The EPA proposes to amend the footnotes for the transient test intervals for spark-ignition and compression-ignition engines in paragraphs (b) and (c), respectively, of Appendix B to 40 CFR part 1036 and the transient duty cycle for compression-ignition engines in paragraph (d) of Appendix B to 40 CFR part 1036 to change them from “Close throttle motoring” to “Minimum operator demand.” This proposed change would alleviate any confusion for engines that do not have throttles. The intent of the motoring points in these test intervals and duty cycles is to operate the engine with minimum operator demand, which results in the throttle being closed for engines that have throttles.

2. Proposed Revisions for Heavy-Duty Vehicles (40 CFR Part 1037)

The existing regulation at 40 CFR 1037.103 establishes evaporative and refueling standards for heavy-duty vehicles. For vehicles using liquefied natural gas, the existing regulation establishes hold-time requirements to limit off-gassing of fuel by referencing SAE J2343. The EPA is proposing to update the reference to the February 2018 version of SAE J2343 for the hold-time requirements that apply to those refueling systems. Also, the existing regulation specifies fuel connection requirements for vehicles using compressed natural gas by referencing the ANSI NGV1 standard in 40 CFR 86.1813-17. The EPA is similarly proposing to update the reference to the version of the ANSI NGV1 standard that was published December 2022.

See the proposed changes to the incorporation by reference provisions in 40 CFR 1037.810 and 40 CFR 86.1.

An earlier final rule included provisions allowing limited numbers of qualified heavy-duty highway vehicles to have certified engines meet alternative standards derived from the EPA's nonroad engine programs.

The provisions apply to amphibious vehicles, vehicles with maximum operating speeds of 45 mph or less, and all-terrain vehicles with portal axles. The provisions also apply to hybrid vehicles with engines that provide energy for an RESS. The EPA added a sunset clause for hybrid vehicles at the end of MY 2027 based on the expectation that greenhouse gas standards would lead to widespread availability of engines certified with hybrid power systems to emission standards under 40 CFR part 1036. Once there are engines certified to heavy-duty highway standards under 40 CFR part 1036 that meet the design requirements of these vehicles, it will no longer be necessary to preserve the option to use engines meeting the alternat

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