Diesel Exhaust Fluid (DEF) failures: EPA Administrator Zeldin demands manufacturer data

EPA Administrator Lee Zeldin has demanded information from major diesel engine manufacturers about DEF system failures and said the agency will use the data to inform rulemaking and actions in 2026.

EPA Administrator Lee Zeldin has demanded detailed information from major diesel engine manufacturers about Diesel Exhaust Fluid (DEF) system failures. The agency said this follows August 2025 guidance that changed derate practices affecting farmers, truckers, and other diesel equipment operators. Under Section 208(a) of the Clean Air Act, EPA is requiring data on warranty claims, failure rates, and repairs for Model Years 2016, 2019 and 2023. Manufacturers representing over 80 percent of DEF products have 30 days to provide the requested information. Key details: - EPA is demanding information from the top 14 on-road and nonroad manufacturers that account for over 80 percent of products used in DEF systems. - The requested data covers warranty claims, failure rates, and repair information for Model Years (MY) 2016, 2019 and 2023 emission control products. - Manufacturers have 30 days to respond; EPA said failure to comply may lead to additional inquiries or penalties. - August 2025 guidance changed derate timelines so heavy-duty trucks receive a longer warning period and milder derates before speed limits apply; EPA said new on-road trucks starting MY2027 must be engineered to avoid sudden severe power loss after DEF depletion. - EPA is working on reconsideration of the 2022 Heavy-Duty Engine and Vehicle NOx rule and said it will use collected data to inform actions in 2026. Summary: The agency says the data will let EPA independently evaluate ongoing DEF system failures and help shape potential rulemaking. Manufacturers have 30 days to provide the requested information. Undetermined at this time.