The U.S. Environmental Protection Agency announced additional measures aimed at addressing ongoing diesel exhaust fluid (DEF) system issues that affect heavy-duty diesel vehicles, including commercial trucking fleets. The action builds on guidance the agency issued in August 2025 related to DEF system malfunctions and engine derates.
According to the EPA, the agency is requiring major diesel engine manufacturers to provide warranty, failure rate, and repair data related to DEF systems for multiple model years. The EPA said the data will be used to better understand the frequency and causes of DEF system failures and to inform potential future regulatory action.
DEF systems are a core component of selective catalytic reduction (SCR) technology used in modern diesel engines to reduce nitrogen oxide (NOx) emissions and meet federal emissions standards. When DEF systems malfunction or register faults, vehicles can experience engine derates, including reduced speed or eventual shutdown, as part of emissions compliance requirements.
In its August 2025 guidance, the EPA modified how quickly and severely derates are required to occur following certain DEF-related faults. That guidance extended warning periods before significant power reductions take effect and introduced a more graduated derate process, rather than abrupt or immediate shutdowns. The EPA said the changes were intended to address reliability concerns while maintaining emissions compliance.
The announcement does not change emissions standards or eliminate derate requirements but signals continued EPA scrutiny of DEF system performance and manufacturer responsibility. The agency said it will use the collected data to evaluate whether additional regulatory updates are necessary.
The EPA emphasized that emissions control requirements remain in place and that SCR and DEF systems continue to play a critical role in reducing NOx emissions from heavy-duty diesel vehicles. Any future regulatory actions, the agency said, would be evaluated in the context of emissions performance and statutory authority.
For commercial trucking fleets operating diesel equipment, the announcement indicates continued federal attention on DEF system reliability and enforcement, as well as the potential for further rulemaking informed by manufacturer data submissions.
