Diesel vehicles would no longer be required to drastically reduce power or limit their speed because of certain diesel exhaust fluid problems under a new proposal from the U.S. Environmental Protection Agency.
The proposal would replace federally required DEF-related engine derates with visible and audible warnings on newly manufactured diesel vehicles and equipment. That means a DEF warning light, bad sensor or suspected emissions-system problem would not automatically turn an otherwise operable truck into a five-mph parade float.
The change would cover a wide range of diesel-powered products, including light- and medium-duty pickups and vans, heavy-duty commercial trucks and nonroad machinery such as tractors and construction equipment. Manufacturers could adopt the new warning-based approach before the 2029 model year, but it would become mandatory no later than model-year 2029 if the rule is finalized as proposed.
DEF is injected into the exhaust stream of most modern diesel engines as part of the selective catalytic reduction system. It helps reduce nitrogen oxide emissions, but the sensors and electronic controls monitoring the system have generated years of complaints from truckers, farmers and diesel pickup owners.
When the system detects an empty DEF tank, poor-quality fluid, possible tampering or a component failure, the vehicle may progressively reduce power and speed. Some vehicles eventually will not restart after being shut off. The system cannot always distinguish between intentional tampering and a failed sensor, potentially sidelining a vehicle even when its engine is otherwise operating normally.
Under the EPA proposal, drivers would receive increasingly persistent warnings telling them to add DEF or repair the system, but EPA emissions regulations would no longer require the vehicle to slow down or become inoperable.
Manufacturers could still reduce engine power when necessary to prevent catastrophic damage to the engine or exhaust aftertreatment system. The proposal is aimed specifically at derates used to compel compliance with DEF and emissions-system requirements.
This does not mean DEF is going away. Diesel vehicles would still be required to meet federal emissions standards, and owners would remain responsible for keeping the emissions system working properly. Manufacturers would also have to monitor DEF supply and quality and warn drivers when attention is needed.
EPA says DEF is now widely available at truck stops, fuel stations and retail stores, making severe engine restrictions less necessary than when selective catalytic reduction systems first entered widespread use. The agency also believes eliminating sudden shutdowns could reduce the temptation for owners to illegally disable emissions equipment.
The proposed regulation builds upon several earlier EPA actions. In August 2025, the agency issued voluntary guidance encouraging manufacturers to provide software updates that give owners substantially more time to correct a DEF problem before severe speed restrictions occur.
In March 2026, EPA issued additional guidance allowing manufacturers to replace or supplement traditional DEF quality sensors with other monitoring technology, including nitrogen oxide sensors. EPA identified DEF quality sensors as having among the highest failure rates of components in selective catalytic reduction systems.
Those earlier actions are important for vehicles already on the road, but they did not completely eliminate derates. The new proposal goes much further by replacing federally required derates with warnings on newly manufactured vehicles and engines.
Owners of existing diesel vehicles should not expect an immediate software update. EPA is considering additional guidance that could allow manufacturers to replace derates with warnings on some in-use vehicles, but the agency acknowledges that developing and testing retrofits may not be practical for every model.
The proposal was published in the Federal Register on July 14, and the public-comment period ended Aug. 29. EPA must review those comments before issuing a final rule, and the provisions could change before becoming effective.
EPA estimates that its entire package of proposed heavy-duty regulatory revisions could produce as much as $12 billion in net savings between 2027 and 2055. However, that figure includes unrelated changes to emissions-system warranty periods and other heavy-duty regulations. It should not be described as $12 billion in savings resulting solely from eliminating DEF derates.
For current diesel owners, the short-term advice remains unchanged: Keep the DEF tank filled, address warning lights promptly and ask the manufacturer or dealer whether an EPA-approved software update is available. For future buyers, however, the days of a faulty DEF sensor bringing a perfectly usable truck nearly to a stop may finally be numbered.