EPA Seeks Comments on Proposed 2027 Heavy-Duty Engine Rules

The Environmental Protection Agency is accepting public comments on proposed changes to rules for model-year 2027 and later heavy-duty highway engines. Written comments are due by 11:59 p.m. EDT on August 29, 2026.

The proposal was published in the Federal Register on July 14, 2026. It is a proposed rule, not a final regulation, so the changes are not yet in effect.

EPA is considering revisions to emissions-related warranty periods, regulatory useful-life requirements, nonconformance penalties for some manufacturers and the way certain diesel exhaust fluid, or DEF, and selective catalytic reduction, or SCR, problems are handled. The docket is EPA-HQ-OAR-2026-0728.

What EPA is proposing

For model-year 2027 and later engines, EPA proposes retaining the emission-related warranty periods that applied to model year 2026 and earlier engines. The proposed minimum warranty periods would be five years and 100,000 miles for medium- and heavy-heavy-duty engines, subject to the applicable regulatory class and whichever limit comes first.

EPA also proposes delaying the longer regulatory useful-life requirements adopted in the 2023 heavy-duty rule by three years, until model year 2030. Regulatory useful life is the period during which an engine must continue meeting applicable emissions requirements for certification purposes; it is not necessarily the same as the engine’s total operating life.

The agency also proposes making nonconformance penalties available beginning in model year 2027 for manufacturers of medium heavy-duty and heavy heavy-duty diesel engines. Under the proposal, eligible manufacturers could temporarily certify and sell engines above an applicable emissions standard while paying prescribed penalties and meeting other conditions. EPA says the provision is intended as a bridge for manufacturers facing technical delays in completing engines designed for the 2027 standards.

A separate section addresses SCR systems that use DEF. For newly manufactured diesel-fueled highway engines and vehicles, as well as certain nonroad engines and equipment, EPA proposes replacing some SCR-related engine derates and vehicle-speed reductions with audible and/or visible notifications.

If finalized, that change could mean future model-year equipment warns operators about specified DEF or SCR problems without automatically reducing engine power or vehicle speed in every situation. EPA says manufacturers could still use derates when needed to protect an engine or aftertreatment system from catastrophic damage.

The proposal principally addresses newly manufactured engines, vehicles and equipment. It would not automatically change the operation of existing trucks. EPA is separately considering guidance that could allow manufacturers to modify some in-use equipment, but that possibility remains under consideration and is not an automatic retrofit program.

What would stay the same

EPA says the proposal would leave the core nitrogen-oxide standards adopted in its 2023 heavy-duty rule in place. The more stringent standards and representative test cycles would remain part of the program.

Using agency modeling, EPA estimates that the amended program would retain nearly 90% of the nitrogen-oxide reductions originally projected under the 2023 rule. EPA also estimates that the amended program would reduce the on-road heavy-duty nitrogen-oxide inventory in 2055 by about 42%, or more than 260,000 tons, compared with a scenario without the 2023 rule. Those are modeled projections, not observed emissions results.

EPA’s analysis also projects higher nitrogen-oxide emissions than under the unchanged 2023 program because of the proposed warranty and useful-life revisions. The actual environmental and operational effects would depend on the final rule, manufacturer compliance decisions, DEF use, SCR reliability and real-world maintenance.

Why the proposal matters

EPA says manufacturers, suppliers, fleets and dealers raised concerns about the cost of the 2023 rule, the longer warranty and useful-life requirements and technical challenges associated with the January 2027 model-year transition.

For manufacturers and fleets, the proposed revisions could affect warranty exposure, compliance costs, engine availability and the timing of new product development. EPA estimates that shorter warranty requirements could reduce vehicle costs, although purchasers could face more repair expenses after the proposed warranty period ends.

For operators, the most visible potential change involves DEF and SCR faults on future equipment. A warning-based system could reduce the risk of an automatic derate or speed reduction for some covered problems, but the proposal does not remove emissions obligations or authorize tampering with emissions controls.

How to submit comments

Manufacturers, trucking companies, equipment makers, environmental organizations, communities near freight corridors and other members of the public may submit written comments through Regulations under docket EPA-HQ-OAR-2026-0728. EPA also lists email, mail and hand-delivery options on its public-comment page.

Comments submitted to the docket may be posted publicly without change, including personal information included in the submission. EPA advises commenters not to place confidential business information or other restricted material in the public docket.

The agency’s virtual public hearings on the proposal have concluded. The immediate public-participation opportunity is the written-comment period, which closes August 29, 2026.

After the deadline, EPA will review the docket and decide whether to revise, withdraw or finalize the proposal. Until a final rule is issued and becomes effective, the proposed changes to warranties, useful life, penalties and DEF/SCR fault responses remain under consideration.

Sources

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