On July 9, 2026, the Environmental Protection Agency (EPA or Agency) proposed revisions to its 2023 Final Rule establishing requirements for Model Year 2027 and later heavy-duty trucks as well as revisions to the regulations regarding selective catalytic reduction (SCR) emissions control systems, while leaving upcoming tailpipe emissions standards in place.

For Model Year 2027 and beyond, the proposed amendments would revert or delay upcoming regulatory advancements regarding useful life and warranty periods and create the option of certification through nonconformance penalties for medium- and heavy-duty vehicle manufacturers who are unable to meet the more stringent emission standards applicable in Model Year 2027 and beyond. In addition, the EPA has proposed to replace engine derating requirements in the SCR inducement regulations with audible and visual signals.

Proposed Revisions to Model Year 2027 Heavy Duty Vehicle Requirements

In proposing revisions to the 2023 Final Rule regarding emissions requirements for Model Year 2027 and beyond heavy duty highway vehicles, EPA is proposing changes in three areas:

• Retain the Model Year 2026 heavy-duty emission-related warranty periods for Model Years 2027 and beyond, rather than use an extended period currently set to come into effect for Model Year 2027;
• Delay the longer useful life periods prescribed by the 2023 Final Rule until Model Year 2030 instead of Model Year 2027; and
• Establish nonconformance penalties under Clean Air Act section 206(g), whereby manufacturers of medium- and heavy-duty diesel engines can obtain EPA certification without meeting the Model Year 2027 standards.

In delaying the extension of the useful life periods until Model Year 2030, the Agency expresses an intent to give manufacturers time to validate their technology’s ability to comply over those longer periods.

Regarding nonconformance penalties (NCP), EPA proposes to move the NCP regulations from 40 C.F.R. Part 86, Subpart L into a new section at 40 C.F.R. Part 1071 devoted exclusively to nonconformance penalties, while also updating the NCP provisions for the Model Year 2027 standards. EPA last created an NCP regime in the 2012 rule for heavy-duty diesel NOx standards. The Model Year 2027 proposal appears to follow a similar approach and penalty formula, while updating numerical values for current technology and including medium-duty vehicles as well as heavy-duty in this iteration.

EPA describes the primary goal of the proposed rule as reducing the cost of new medium- and heavy-duty vehicles. However, the Agency asserts that most truck and engine manufacturers have already planned a Model Year 2027 lineup that meets the 2027 standards and are “likely to maintain a steady design technology for MY 2028 and 2029” rather than redesign based on the proposed useful life revisions. For any manufacturers not currently able to meet the Model Year 2027 standards, the statutory language regarding nonconformance penalties requires that the penalty formula eliminates any competitive advantage associated with the nonconformance.

In the proposed rule, EPA asserts that, 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.” The Agency identified the proposed reversion of warranty length as the theoretical source of any upfront vehicle cost savings to consumers. The agency concedes that much of this upfront savings would be consumed by increased operating costs.

Proposed Revisions to SCR Inducement Regulations for Heavy Duty Vehicles and Engines

The Agency also proposes to “replace engine performance derates as a part of SCR-related inducement provisions with visible and/or audible notifications” in both highway and nonroad applications. Manufacturers would still have to monitor for SCR-related faults and generate audible or visible signals for certain SCR system conditions. The proposed rule describes continuous visible signals and a schedule of 90-second audible signals, which varies depending on the condition detected.

EPA proposes a transition structure under which the existing derate-based inducement framework may continue through Model Year 2028, with the new audible-warning approach taking effect thereafter. The regulation redline includes special transition provisions that preserve the current speed-derate schedule during that period. But eventually, under the proposal, mandatory emissions-related derates would largely disappear for newly certified engines.

Some operators could still experience engine derate events under the Agency’s proposed revisions if manufacturers determine that they are necessary for hardware protection. As Truck and Engine Manufacturers Association President and CEO Jacqueline Gelb recently testified to the House Subcommittee on Environment, diesel exhaust fluid (DEF) technology is fully integrated into engine systems and the full elimination of DEF from the system could cause catastrophic equipment failures.

The proposed rule also includes new SCR design requirements accompanying the inducement changes. Manufacturers would be required to use DEF-quality compensation algorithms that maintain NOx control despite varying DEF quality and provide DEF freeze-protection systems capable of restoring DEF flow within specified timeframes.

Proposed Rule Echoes Agency Retreat from Tampering Enforcement

Under the second Trump administration, EPA and DOJ have taken numerous steps to limit federal enforcement of the Clean Air Act’s prohibition against vehicle tampering, which was the subject of an EPA enforcement initiative designated by the first Trump Administration and primarily implemented by the Biden EPA.

In January 2026, in a pair of internal memos, EPA and DOJ each adopted a policy of no longer pursuing criminal enforcement of vehicle tampering. On June 29, 2026, the White House issued a Presidential Memorandum directing EPA to, among other things, “consider deprioritizing civil tampering enforcement actions against anyone who, in good faith, attempts to fix his or her own vehicle to its original configuration.” The Agency responded by issuing guidance supporting vehicle owners’ Freedom to Fix their vehicles. This proposed rule furthers the Agency’s retreat from tampering prevention.

In prior administrations, EPA sought to extend vehicle warranty periods in order to decrease both consumer maintenance costs and the likelihood that consumers will engage in illegal aftermarket tampering as a response to emissions system failures. Similarly, the inducement regulations are intended to ensure appropriate use of vehicle emission controls. With shorter warranty periods and no engine derating for failure to use required DEF, there is likely to be a significant decline in proper emission control system operations for in-use vehicles.

Federal Approach to In-Use Compliance May Inspire State Involvement

While federal interest in tampering enforcement and the bolstering of in-use emissions compliance is waning, the same cannot be uniformly said of state governments. Unlike the ongoing legal battle between the Trump administration and the California Air Resources Board regarding California’s emissions standards for new vehicles, state authority over in-use vehicles is not preempted by the Clean Air Act.

Transportation is a major contributor to NOx pollution, and in particular EPA estimates that each deleted diesel pickup truck contributes more than a ton of extra NOx pollution. As the fight over new vehicle standards draws out in the courts, there may be a heightened focus on in-use emissions by states interested in addressing mobile source pollution.

California and other states may pursue compliant in-use vehicle emissions through continued state enforcement against aftermarket tampering, heightened scrutiny on emission controls in vehicle inspection and maintenance programs, or the expansion of roadside emissions monitoring initiatives. At a minimum, state inspection and maintenance programs are likely to require drivers to address any audible or visual signals of SCR problems before registration. Any software disabling those signals could amount to vehicle tampering.

Next Steps

The EPA will hold public hearings on the proposed rule on July 29 and 30, and the Agency will receive public comment on the proposed regulations through August 29, 2026.

A final rule will not be issued by the Agency until sometime thereafter, at which point some manufacturers will likely have already received their Model Year 2027 certification and most manufacturers will have set their strategies for not only emissions but also for pricing for Model Year 2027. Any manufacturers wishing to avail themselves of the NCP provisions will have to wait until the rule is finalized to do so.

In planning for Model Years 2028 and beyond, manufacturers would be wise to consider how their chosen approach prepares them for future changes and political uncertainties.

For More Information

Van Ness Feldman closely monitors and counsels clients on energy transition and air quality law and policy, including the Clean Air Act. For further details or assistance with assessing the impacts of this final rule, please contact Britt Speyer Fleming or any member of VNF's Environmental Team.