The Environmental Protection Agency has released a 477-page proposed rule for amendments to certain compliance provisions and test procedures related to model year 2027 and later heavy-duty highway engines. These proposed amendments would include revisions to the emission-related warranty and the regulatory useful life for all heavy-duty engines, and improved access to the five percent production volume allowance for heavy-duty engines finalized in January 2023.

The proposal would not change the NOx standards first promulgated in the Biden Administration. “The more stringent standards and more representative test cycles finalized in the 2023 Final Rule remain in place,” the agency stated. 

The proposal would also correct or clarify test procedures and other compliance provisions adopted in January 2023 and in earlier rules. The EPA also proposed making nonconformance penalties available to manufacturers of medium heavy-duty engines and heavy heavy-duty engines beginning in MY 2027. This would allow an engine manufacturer to pay a monetary fine for failing to meet NOx standards.

Additionally, the EPA is proposing to replace SCR-related engine performance derates and vehicle speed reductions with audible and/or visible notifications, along with other actions to improve DEF system reliability. 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 elimination of deratements, where an engine suddenly slows or stops if its emissions systems are signaling an issue with their operations, would be a significant benefit to all truck operators. Automatic deratement would be replaced with a 90-second visual or audible warning that a truck’s emission systems are failing in some manner, including a lack of diesel exhaust fluid in the system.

In August 2025, EPA issued new guidance asking engine and equipment manufacturers to revise DEF system software in existing vehicles and equipment to reverse deratements. This new proposal would completely remove deratements and vehicle speed restrictions for newly manufactured highway engines and vehicles and new nonroad engines and equipment, including those used in agriculture. Instead of trucks and tractors lurching to a halt when DEF systems fail, under EPA’s proposal, operators would receive visible and/or audible alerts that allow them to continue operating until they can safely address the problem.

EPA is also accepting 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 agency has scheduled a virtual public hearing on the proposal 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.

The proposed rule would also reduce the required mileage for engine warranties to 100,000 miles and would not go into effect until MY 2030.

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

An EPA statement said savings from the rule, if finalized, could reach up to $6,000 per vehicle on new truck purchases, on top of the increased productivity operators are currently losing when deratements cause sudden speed loss on the road.