Farmers frustrated with diesel exhaust fluid (DEF) shutdowns may see fewer breakdowns under new Environmental Protection Agency (EPA) guidance, though the agency’s new guidance on DEF stopped short of eliminating the emissions systems altogether.
EPA did not eliminate the DEF rules, which have been a problem for farmers because of failure rates, but issued new guidance, giving major machinery manufacturers more flexibility in how they monitor exhaust emissions. EPA will now allow equipment manufacturers to use nitrous oxide (NOx) sensors to reduce problems with tractors and semi-trucks “de-rating” during operations.
Farmers will also have more opportunities to make repairs on the DEF sensors under EPA’s guidance.
Speaking to farmers at a White House celebration on March 27, President Donald Trump called DEF restrictions “ridiculous,” saying he would only buy older equipment for golf courses that didn’t have them.
“Do you know about that? It was a basic disaster. Today, we’re announcing new guidelines, drastically limiting these ridiculous DEF rules, saving farmers and consumers billions and billions of dollars,” Trump said.
Trump said EPA’s move also would significantly lower costs for farmers, or he would come after the equipment manufacturers.
The Small Business Administration (SBA) claims the EPA changes will save farmers $4.4 billion a year in repairs and lost productivity, though last August, SBA said the regulatory change would save farmers $727 million per year in downtime and repairs. SBA also said the change would provide nearly $13.8 billion in savings nationally.
Spelling out the specifics
Since 2013, EPA has required diesel engines to have selective catalytic reduction (SCR) emission controls and required DEF as the fluid that makes SCR work to reduce nitrous oxide emissions.
To comply, most manufacturers began installing urea quality sensors, which have become a major source for failures, leading to engine stalls and derating of power and shutdowns. Groups such as the Truck and Engine Manufacturers Association have identified the urea quality sensors as having among the highest failure rates of any equipment used in emission controls.
EPA said a preliminary review of warranty data suggests that DEF sensor failures are a significant source of warranty claims. “Farmers and truck drivers should not have their vehicles stop working because a sensor isn’t working properly,” EPA stated.
Under the new guidance, manufacturers can reduce or end DEF sensor failures by removing urea quality sensors and replacing them with NOx sensors. EPA will also allow NOx sensors to be installed on existing engines without it being considered illegal tampering.
Patrick Kelly, vice president of energy and environmental affairs for the American Trucking Association, said EPA offered a pragmatic solution by allowing more flexibility on DEF monitoring to prevent forced shutdowns.
“As we have previously said, these systems have too often sidelined otherwise safe, compliant trucks due to faulty or unreliable DEF quality sensors, an issue that was compounded by widespread parts shortages in recent years,” Kelly said. “When a bad sensor can trigger a full inducement, the result is unnecessary downtime, unnecessary towing costs, strained supply chains, and higher costs across the board.”
In August 2025, the Trump administration announced new guidance that allows farmers and truckers to revise software to prevent sudden speed and power losses caused by DEF system faults.
Starting with model year 2027, that guidance requires all new diesel on-road trucks to be engineered to avoid sudden and severe power loss after running out of DEF.
What the change means
EPA is now saying farmers can restart their equipment if it has a DEF issue. In the past, EPA rules often required that if a farmer had a deratement issue, they would need to have the original equipment dealer come out and reset the machine.
That fix could still come with a cost, such as an annual licensing fee. For instance, a farmer still might need a software update on their equipment to allow them to repair it themselves. Companies such as Deere & Co. have software updates with their John Deere Operations Center Pro Service, so a farmer can fix it and not have to wait for the dealer. That Pro Service, however, requires an annual licensing fee. — Chris Clayton, DTN ag policy editor
