The United States Environmental Protection Agency is on a roll when it comes to undoing past regulations and directives. Now, EPA Administrator Lee Zeldin is urging truck and engine manufacturers to do away with sudden speed and power losses due to low diesel exhaust fluid levels. In his announcement at the Iowa State Fair, a key event humming with agricultural workers who have long voiced their negative opinions of modern diesel emissions systems, Zeldin called the software-imposed restrictions “unacceptable” and “unnecessary.”
There certainly would have been a time when such a move was surprising for the EPA, but the Trump Administration is conducting an about-face from its predecessors on environmental matters. It was Zeldin who, less than two weeks ago, announced the agency’s intentions to stop regulating greenhouse gas emissions. He claimed that as a victory for industry, while this latest decision aims to champion everyday drivers and equipment operators.
“We have heard loud and clear from small businesses across the United States that the current DEF system is unacceptable. It is unacceptable that farmers, truckers, construction workers, and many other small businesses continually experience failures of diesel-powered equipment when they need it most—costing millions of dollars in lost productivity,” Zeldin insisted. “Today, we are responding to those concerns by calling on manufacturers to take action to update their software and eliminate the unnecessary sudden loss of power and frustrating shutdowns that too many Americans have experienced.”
To explain it quickly, in case you aren’t familiar: Most modern diesel engines rely on DEF for a process called selective catalyst reduction (or SCR). This fluid, which is a solution made from urea and deionized water, is injected into the exhaust stream to reduce nitrogen oxide (NOx) emissions by as much as 99%. It’s used in everything from heavy-duty pickup trucks to semis, tractors, excavators, and so on.
While DEF serves the clear purpose of improving emissions, many people malign it and everything that comes with it. The federal government estimates that hundreds of thousands of drivers have removed their trucks’ emissions equipment altogether to avoid using DEF, citing performance and reliability concerns. Such modifications have been the cause of major fines and penalties dished out to diesel aftermarket companies in recent years.
For their part, manufacturers have made DEF a non-negotiable part of engine operation to meet restrictions put in place by past versions of the EPA. Even though DEF-related “limp mode” was never mandated by the federal government, companies use the method to prevent people from running their vehicles and equipment without the solution.
New EPA requirements taking effect in model-year 2027 state that new diesel trucks for road use cannot be engineered to force sudden and severe power loss after running out of DEF. Additionally, the EPA says it’s issuing new guidance that was “developed in collaboration with manufacturers” to make the necessary software changes for vehicles already in use. It’s a sweeping move, then, applying to both used and new trucks.
And to be clear, engine derating will still be part of the strategy, but to a far lesser degree. Zeldin explained in an op-ed for the Des Moines Register that on heavy-duty semis, drivers will still be able to run for a full day with low DEF levels before performance is restricted slightly by 15%. Top speed, meanwhile, won’t be limited for much longer; only after about four work weeks will semis be limited to 25 mph.
It’s a similar story with heavy-duty pickups, which will only be limited to 45 mph after 4,200 miles or 80 hours with low DEF levels.
The Drive has reached out to Ford, General Motors, and Ram for comment. I’ll update this story when or if they respond. One way or another, this is big news that will surely prove to be polarizing in due time. What’s new there, though?
Updated at 1:20 p.m. ET on August 12, 2025: This story now includes more details about the EPA’s plan to eliminate sudden power loss, instead making the engine derating process more gradual for drivers and equipment operators alike.
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