US EPA proposes to relax 2027 emission requirements for heavy-duty engines
13 July 2026
The US Environmental Protection Agency (EPA) released a proposed rule to relax certain provisions of the 2027 emission requirements for heavy-duty highway engines and vehicles, including emission useful life periods, emission warranty periods, and driver ‘inducement’ requirements for vehicles equipped with urea-SCR aftertreatment. The proposal also introduces nonconformance penalties that would allow manufacturers to temporarily continue sales of non-compliant engines.
The proposal maintains the underlying emission standards, including a nearly 90% reduction of NOx emissions, as specified in the original regulation adopted in December 2022 and published in the Federal Register in January 2023.
According to EPA estimates, the proposal, if adopted, could save up to $6,000 per vehicle on new truck purchases and eliminate the productivity losses operators are experiencing when SCR-related engine deratements cause sudden speed loss on the road. Collectively, American truckers would save 12 billion through the proposed revisions.
Under the proposal, the EPA is withdrawing the lengthened emission-related warranty periods introduced by the 2023 regulation. The proposed rule shortens the warranty periods for model year (MY) 2027 and later engines to the values that apply to MY 2026 and earlier engines. In the 2023 rule, the projected costs due to the lengthened emission warranty periods were the largest contributor to the projected per-vehicle cost increases and total program costs.
The EPA is also proposing additional lead time before the longer regulatory useful life periods take effect. The MY 2027 and later engines’ useful life periods are to be delayed to MY 2030. As such, the current MY 2026 and earlier useful life periods would continue to apply through MY 2029.
Under the proposal, the EPA is continuing the existing allowance for manufacturers to produce up to five percent of their US-directed production volume of heavy heavy-duty engines (Heavy HDE, >33,000 lbs) that is compliant with pre-2027 requirements, but proposes to remove the requirement to use NOx credits.
The EPA is proposing nonconformance penalties (NCP) for the Heavy HDE and Medium HDE categories that temporarily cannot meet the new NOx standards. The NCPs are intended to provide flexibility to comply and allow sales to continue, helping prevent supply disruptions and supporting a stable transition to new technologies.
Manufacturers using NCPs would go through the certification process, submitting data showing that the engine complies with emission standards, except that NOx emissions may exceed the applicable standard but can be no higher than the upper limit of 200 mg/hp·hr, which is the NOx standard that applies to MY 2026 engines. Depending on the NOx compliance level, the NCPs can range from zero to about $4,300 per engine for Medium HDEs and up to about $6,800 for Heavy HDEs.
Finally, the proposed rule addresses concerns from truckers and other diesel equipment operators about SCR-related inducement, i.e., strategies—such as engine derates and vehicle speed reductions—to induce operators to maintain appropriate levels of high-quality diesel exhaust fluid (DEF) and not tamper with SCR systems. In the 2023 final rule, the EPA codified requirements that describe how manufacturers can design engines with a derate schedule to demonstrate that operators would be reasonably likely to maintain a supply of quality DEF. For instance, the regulation specifies an immediate initial restriction in allowable vehicle speed and further decreases in allowable speed over time until a final inducement speed of 25 mph is reached.
Now, the EPA is proposing to replace engine performance derates for SCR-related inducements with visible and/or audible notifications. Under the proposal, a 90-second audible notification would be required when a DEF refill becomes necessary. The first inducement notification will start three hours prior to the tank becoming empty or when the tank is at 2.5% full. The notification would be then repeated 30 and 60 minutes after empty DEF level, and finally every hour. For DEF quality- and tampering-related failures, the EPA is proposing a less frequent audible notification schedule.
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. The EPA is also taking public comment on whether guidance should be developed to allow manufacturers to implement this change for in-use on road and nonroad engines and equipment.
The proposed changes to SCR inducement follow earlier actions by the agency. In February, the EPA requested data on DEF system failures from engine manufacturers. In March, it relaxed the guidance on the DEF quality monitoring in SCR systems.
The EPA will hold a 45-day public comment period as well as a public hearing on the proposed rule.
Source: US EPA