WASHINGTON — In a move to end “limp mode” nightmares for truckers, Environmental Protection Agency Administrator Lee Zeldin announced on Tuesday that the agency is launching a formal investigation into the reliability of Diesel Exhaust Fluid (DEF) systems.
EPA is demanding that the top 14 diesel engine manufacturers – representing 80% of the market – hand over internal data on warranty claims and engine failure rates, with the goal of determining why DEF systems often fail and to hold manufacturers accountable for hardware that can leave trucks stranded. The information will be used as the basis for a future rulemaking.
The investigation is part of a broader initiative at EPA to treat issues related to DEF – used to reduce nitrous oxide emissions – as a driver compliance problem to a manufacturer quality problem.
In August, the Trump administration issued guidance that effectively ended the immediate 5-mph engine “derate” for most DEF-related faults.
Before that change, a faulty sensor could leave a driver stranded within hours. Under the current 2025 protocols, diesel engines now give drivers a 650-mile or 10-hour warning window before a mild engine derate occurs, with a full speed restriction to 25 mph not kicking in for nearly four work weeks.
“Today, we are furthering that work and demanding detailed data to hold manufacturers accountable for the continued system failures,” Zeldin said in a press release. “The Trump EPA is committed to ending unnecessary frustrations and days lost on the road and in the field for American farmers and truckers.”
EPA is specifically targeting warranty claims, DEF failure rates, and repair data from Model Years 2016, 2019, and 2023. By comparing these model years, the agency aims to identify if specific components or designs are prone to premature failure. Manufacturers have 30 days to comply or face “additional inquiries or penalties,” EPA warned.
The agency is also reconsidering the 2022 Heavy-Duty Engine and Vehicle NOx rule. “It is being thoroughly assessed whether derates may no longer be necessary for compliance,” EPA stated.
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- How Diesel Exhaust Fluid is Impacting the Trucking Industry
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