The Environmental Protection Agency is issuing guidance to manufacturers for how they may modify Diesel Exhaust Fluid systems on new and existing diesel vehicles and nonroad equipment to reduce derates that can limit a vehicle's performance to nearly inoperable levels (as low as five miles per hour).  The agency is urging engine and equipment manufacturers to follow this guidance and develop software updates that align with these recommendations.

Since 2010, nearly all on-road diesel trucks and many types of nonroad equipment (such as tractors and construction machinery) have used DEF as part of selective catalytic reduction systems to reduce emissions of nitrogen oxides, a major contributor to air quality nonattainment. These systems can force a vehicle to drastically reduce speed or become inoperable if DEF runs out or if system sensors fail.

Although this derate strategy was intended to ensure compliance with EPA's Tier 4 Emissions Standards, EPA asserts it has caused needless frustration, operational delays, and real economic hardship for farmers, truckers, and equipment operators. The agency states it is taking this action in response to ongoing feedback from operators, farmers, and industry stakeholders about real world impacts of diesel engine derates. By issuing guidance and partnering with manufacturers to act, EPA says it is working to ensure emissions compliance while minimizing disruptions to safety, transportation, and commerce.

EPA requires vehicles using DEF to meet specific emissions standards, particularly for NOx. These requirements include:

• Emissions Limit Requirements: Vehicles must effectively meet EPA Tier 2, Tier 4, or 2010 emissions standards, depending on the vehicle type and model year.

• On-Board Diagnostics: Vehicles must have OBD to monitor the SCR system to detect low-DEF levels, system failures, and alert the driver via a warning light.

• Tamper Prevention System: The system must be designed to prevent tampering or bypassing DEF usage.

By law, EPA cannot mandate field fixes for in-use vehicles and nonroad equipment, which is why the agency is issuing voluntary guidance to manufacturers on system modifications to reduce derates.

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. To fix the problem for vehicles already in use, EPA's new guidance, developed in collaboration with manufacturers, will work to ensure that the necessary software changes can be made on the existing fleet. In addition to providing certainty to manufacturers about how EPA wants this issue resolved, the agency is not requiring separate approvals beyond that provided in EPA's guidance.

Additional information, background, and the guidance document are available here.

As anticipated, the California Air Resources Board has also issued guidance authorizing the same derate schedule as EPA, and reports indicate that the engine manufacturers have agreed to implement the guidance with software updates in the coming months.