The Environmental Protection Agency has issued revised guidance to address widespread concerns from diesel vehicle operators about a loss of speed and power, or engine derates, caused by selective catalytic reduction systems that use diesel exhaust fluid.

Specifically, EPA is issuing new 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 nearly inoperable levels (as low as five miles per hour). EPA is urging engine and equipment manufacturers to follow this guidance and develop software updates that align with the agency’s recommendations.

Since 2010, nearly all on-road diesel trucks and many types of nonroad equipment (such as trucks, 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 for on-highway and nonroad vehicles, it has caused needless frustration, and operational delays.

The new guidance letter clarifies that alternative methods of meeting the adjustable parameter provisions, including the use of NOx sensors, are allowed under EPA regulations. Further, the guidance provides an illustrative example of how a manufacturer may make the necessary demonstrations to certify NOx sensor-based systems. Finally, the EPA is stating that software updates to implement one or more elements of this guidance may be installed in existing diesel engine products in place of, or to augment, existing Urea Quality Sensors-based systems without such action being considered tampering under the Clean Air Act.

In addition, in February 2026 EPA announced it was working on a new rulemaking to provide DEF relief. The agency has sought information from major diesel engine manufacturers on data from DEF system failures. The data collected will allow EPA to independently evaluate ongoing system failures and help inform next steps the agency takes in 2026 pertaining to DEF.