The Environmental Protection Agency (EPA) followed up on August 2025 guidance with further heavy-duty diesel truck changes, and demands toward manufacturers.
Back in August, the EPA under administrator Lee Zeldin revised the agency’s guidance for DEF (Diesel Exhaust Fluid) systems. Specifically, that order told manufacturers of heavy-duty trucks to stop forcing immediate limp mode when vehicles ran low on DEF, from model year 2027 forward. However, new guidance announced this week aims to take that a step further.
Now, arguing on behalf of “truck drivers, farmers, and many others rightly complaining about DEF and pleading for a fix”, Zeldin says in the agency’s release, the EPA is stipulating manufacturers engineer their trucks to avoid sudden derating and immediate, severe power loss after trucks run out of DEF from MY 2027 onward.
In addition, under Section 208(a) of the Clean Air Act, the agency is demanding the top 14 on-road and nonroad diesel manufactuers provide data on warranty claims, failure rates and repair information to assess the scale of DEF-related issues in multiple generations of diesel trucks. Specifically, the EPA is demanding automakers turn over data on model years 2016, 2019 and 2023. They have 30 days to do so, or those manufacturers could face additional inquiries or financial penalties for failing to provide information.
“The agency is committed to working with manufacturers to ensure practical, durable solutions that simultaneously support emissions reductions and reliable operations,” the agency said in its official release. It also said it is working on a proposal for the “reconsideration” of the EPA’s 2022 Heavy-Duty Engine and Vehicle NOx rule. The agency continues in its statement: “It is being thoroughly assessed whether derates may no longer be necessary for compliance.”
Under Zeldin’s August 2025 order, on-road heavy-duty trucks should receive a warning light for 650 miles or 10 hours after a DEF fault is detected. Following that, the engine will only “mildly derate”, otherwise operating normally and without speed limits for 4,200 miles or two work weeks. Only after four weeks, trucks would be governed to 25 miles per hour until repairs are made. Nonroad diesel equipment, on the other hand, would see no impacts from running out of DEF or a mechanical failure of the DEF system for the first 36 hours. After that, the equipment would see a “slight torque reduction”. (Again, this is all from MY 2027 forward.)
Beyond the latest guidance saying MY 2027 and later diesel vehicles should have no power loss from running out of DEF, the EPA also issued another piece of guidance to manufacturers this week. That guidance, issued Monday, noted that “Americans have the right to repair their farm and other nonroad diesel equipment” under the Clean Air Act. “This guidance will allow famers to fix broken DEF systems at home or in the fields, saving them time and money, while the agency continues its work on DEF”.
We’ve heard several complaints over the years from truck owners regarding DEF systems, and some of the guys have experienced issues firsthand with their own modern trucks (see below). This new plan could change things long-term for truck owners — particularly if they don’t have to contend with deratements for running out of DEF, and aren’t forced to refill the system to get the maximum performance from their trucks.
