EPA Proposes Easing 2027 Truck Engine Rules, Ending DEF Derates on New Engines

In July 2026, the Environmental Protection Agency proposed significant changes to the heavy-duty engine rules scheduled to take effect with model year 2027. EPA Administrator Lee Zeldin announced the proposal July 9, 2026, at an event on the National Mall, and it was published in the Federal Register on July 14.

The proposal would not change the tighter nitrogen oxide standard adopted in the January 2023 final rule, which sets a limit of 35 milligrams per horsepower-hour for medium and heavy heavy-duty engines. Instead, it targets the compliance provisions that EPA says drove up the cost of new trucks.

What EPA proposed

• Warranty: Emission-related warranty periods for model year 2027 and later engines would stay at model year 2026 levels. The American Trucking Associations described that as the current 5-year, 100,000-mile warranty. EPA said the longer warranties in the 2023 rule were the largest single contributor to projected per-vehicle cost increases.
• Useful life: The longer regulatory useful-life periods would begin in model year 2030 instead of 2027. For heavy heavy-duty engines, the proposed rule text keeps 435,000 miles through model year 2029 and moves to 650,000 miles, 11 years or 32,000 hours starting in 2030.
• Nonconformance penalties: Makers of medium and heavy heavy-duty diesel engines could sell engines that do not meet the 2027 NOx standard if they pay a penalty. EPA said some manufacturers' engine programs had run into technical problems and a few engine families were at risk of missing a January 2027 introduction.
• DEF derates: For newly manufactured highway and nonroad diesel engines, engine power derates and speed limits tied to diesel exhaust fluid and SCR problems would be replaced with visible or audible alerts. EPA said it is also considering guidance on whether manufacturers could apply similar changes to engines already in service.

EPA estimated purchasers could save up to $6,000 per diesel vehicle from warranty changes alone, with net savings of up to $12 billion. The agency said the 2023 rule, as amended, would keep nearly 90% of the NOx reductions originally projected. Comments were due August 29, 2026, with virtual public hearings set for July 29 and 30.

Industry reaction

ATA President and CEO Chris Spear, who appeared with Zeldin, said in a statement that "new environmental standards must be achievable, affordable, and reliable."

The Owner-Operator Independent Drivers Association was more skeptical. Jay Grimes, OOIDA's director of federal affairs, told Overdrive the proposal "marks a missed opportunity to adequately correct the shortcomings of the 2023 Final Rule." He said "sensible warranty programs are critical for encouraging adoption of newer trucks," though he called the longer testing timelines and derate flexibility helpful.

The Truck and Engine Manufacturers Association said it was still reviewing the proposal and urged EPA to finalize it by the end of the year. Calstart, a clean transportation group, warned that the changes could create uncertainty and increase pollution near freight corridors.

What it means for drivers and fleets

Nothing changed immediately; the proposal still had to go through comment and a final rule. If finalized as written, buyers of model year 2027 trucks would not pay for the longer warranty coverage built into the 2023 rule, but they would also not get that extended coverage. Fleets and owner-operators should weigh standard warranties against the cost of extended coverage when specifying new trucks.

The derate change would apply only to newly built engines, so trucks already on the road would still depend on manufacturer software updates under existing EPA guidance.