In a sharp shift from past policies, the U.S. Environmental Protection Agency (EPA) is calling on truck and engine manufacturers to eliminate abrupt speed and power reductions triggered by low diesel exhaust fluid (DEF) levels. Administrator Lee Zeldin made the announcement at the Iowa State Fair, a venue well-attended by farmers and equipment operators who have long criticized modern diesel emissions systems.
Labeling the software-imposed restrictions “unacceptable” and “unnecessary,” Zeldin said the change will help farmers, truckers, and construction workers avoid costly downtime caused by DEF-related shutdowns.
A Break from Previous EPA Stance
Once known for tightening emissions rules, the EPA under the Trump Administration has taken a dramatically different approach. Just two weeks ago, Zeldin announced the agency’s plan to halt regulation of greenhouse gas emissions a decision hailed as a win for industry. The latest move, he said, is about supporting everyday operators of diesel-powered equipment.
“We’ve heard loud and clear from small businesses across the country that the current DEF system is unacceptable,” Zeldin stated. “It’s costing millions of dollars in lost productivity. Today, we’re responding by urging manufacturers to update their software and end sudden power losses and frustrating shutdowns.”
Understanding DEF and Its Role
DEF is a mixture of urea and deionized water used in selective catalyst reduction (SCR) systems. By injecting the fluid into the exhaust stream, SCR can reduce harmful nitrogen oxide (NOx) emissions by up to 99%. The technology is standard in heavy-duty pickups, semis, tractors, and construction equipment.
While DEF effectively cuts emissions, it has been controversial. Many drivers complain about system failures and costly repairs, while some have resorted to removing emissions equipment entirely a practice that has led to hefty federal fines for aftermarket companies.
New Guidelines for 2027 and Beyond
Under updated EPA requirements taking effect for model-year 2027, new on-road diesel trucks cannot be engineered to impose sudden and severe power loss once DEF runs out. In addition, the agency is issuing guidance, developed with manufacturers, for retrofitting existing vehicles to soften DEF-related restrictions.
The revised rules still allow for engine derating, but the impact will be far less severe. For heavy-duty semis, low DEF levels will trigger only a 15% power reduction after a full day’s operation. Speed limits will apply only after about four work weeks, capping at 25 mph.
Heavy-duty pickups will see similar changes, with a reduced speed of 45 mph applying only after 4,200 miles or 80 operating hours with low DEF.
Industry Impact and Operator Relief
The move is expected to be welcomed by operators in agriculture, freight, and construction, where unplanned downtime can derail schedules and eat into profits. Manufacturers will now have to adjust software strategies that were once considered essential for meeting emissions standards.
For many, the change represents a compromise keeping emissions controls in place while reducing the operational headaches that have plagued diesel users for more than a decade.
