New EPA action targets diesel engine derates

Background

When DEF runs out or a system sensor fails, current systems can force a vehicle to drastically reduce speed or become inoperable. In some cases, vehicles are limited to as little as five miles per hour within hours of a DEF-related fault, reportedly causing disruptions in logistics, agriculture, and construction.

Although this derate strategy was intended to ensure compliance with EPA’s Tier 4 Emissions Standards, it has led to reports of operational challenges for farmers, truckers, and equipment operators.

Since 2010, most on-road diesel trucks and many types of nonroad equipment (such as tractors and construction machinery) have used selective catalytic reduction (SCR) systems that inject DEF into exhaust streams to reduce nitrous oxide (NOx) emissions.

Starting with model year 2027, EPA requirements state that all new diesel on-road trucks must be engineered to avoid sudden and severe power loss after running out of DEF. For vehicles already in use, EPA’s new guidance addresses software changes that can be made on the existing fleet.

Details of the action

EPA is issuing guidance to manufacturers for how they may modify DEF systems on new and existing diesel vehicles and nonroad equipment to reduce derates that can limit a vehicle’s performance to levels as low as five miles per hour. The agency is recommending that engine and equipment manufacturers follow this guidance and develop software updates that align with the agency’s recommendations.

The agency is not requiring separate regulatory approvals beyond those provided in the EPA guidance document. Please see EPA's Diesel Exhaust Fluid website for more information.