As a follow-up to new guidance issued in August 2025 that reversed deratements harming motorcoach operators and truckers, US Environmental Protection Agency (EPA) Administrator Lee Zeldin announced that the EPA is seeking information from major diesel engine manufacturers on critical data on Diesel Exhaust Fluid (DEF) system failures.

The data collected will allow EPA to independently evaluate ongoing system failures and inform the agency's next steps in 2026 regarding DEF.

“As I traveled to all 50 states during my first year as EPA Administrator, I heard from truck drivers, farmers, and many others rightly complaining about DEF and pleading for a fix. EPA understands this is a massive issue, which is why we have already established commonsense guidance for manufacturers to update DEF systems,” said Administrator Zeldin. “Today, we are furthering that work and demanding detailed data to hold manufacturers accountable for the continued system failures. The Trump EPA is committed to ending unnecessary frustrations and days lost on the road and in the field for American farmers and truckers.”  

DEF Issues Facing Operators

The issue of motorcoach operators, truck drivers, and farmers remains clear to the agency: sudden speed loss and shutdowns caused by DEF system failures compromise safety and productivity.

The EPA’s guidance, which significantly reversed deratements, called on manufacturers to revise DEF system software in existing fleets to prevent these sudden shutdowns and give operators more time to repair faults. EPA recognizes that, to improve systems, the product designs and materials used in these systems must also be of the highest possible quality.

With this action, EPA is taking another step to address potential deficiencies in system components, advance future rulemaking, and reduce system failures.  

Reasons for EPA’s Step

Under Section 208(a) of the Clean Air Act, EPA is authorized to require manufacturers to provide information needed to assess whether emission control systems are functioning properly and whether manufacturers are meeting their obligations to identify and fix defects. EPA is requesting information from the top 14 on-road and nonroad manufacturers, which account for over 80% of all products used in DEF systems.

Specifically, EPA is requiring data on warranty claims, failure rates, and repair information for Model Years (MY) 2016, 2019, and 2023 emission control products to determine whether ongoing DEF system failures are related to a specific generation of products. Manufacturers will have 30 days to provide the requested information; failure to do so may result in additional inquiries or penalties, according to the EPA. 

The agency said it is committed to working with manufacturers to ensure practical, durable solutions that simultaneously support emissions reductions and reliable operations. EPA has been meeting with manufacturers to receive updates on their progress in developing improved software in response to the agency’s guidance. The agency has consistently encouraged manufacturers to provide the relief as quickly as possible.  

EPA is actively working on its proposal for the reconsideration of the 2022 Heavy-Duty Engine and Vehicle NOx rule. It is being thoroughly assessed whether derates may no longer be necessary for compliance. The agency is committed to keeping the protection of air quality and human health at the forefront of every action it takes, officials said.

Adding to Past Actions

In August, the EPA issued new guidance urging diesel engine manufacturers to update DEF system software to prevent sudden speed and power losses that have disrupted motorcoach operations and raised safety concerns.

Announced by Administrator Zeldin, the guidance gave operators more time to address DEF-related faults before derating occurs and aligns with new EPA requirements taking effect in 2027.

Motorcoach operators and industry groups welcomed the move as a long-sought safety and operational improvement, the result of years of advocacy to address the real-world impacts of abrupt derates on passenger transportation.