EPA issues updated diesel exhaust fluid sensor guidance
On March 27, the Environmental Protection Agency (EPA) issued new guidance regarding diesel emission systems, removing a longstanding requirement for urea quality sensors in diesel exhaust fluid (DEF) systems and allowing manufacturers to rely upon nitrogen oxide (NOx) sensor technology instead.
A March 27 EPA press release states the updated guidance is part of the association’s efforts to address nationwide concerns from farmers, truckers and other diesel equipment operators regarding sudden speed losses and shutdowns caused by DEF sensor failures.
The updated guidance is expected to save farmers $4.4 billion a year in repairs and lost productivity and provide $13.79 billion in annual savings to Americans, according to the U.S. Small Business Administration.
New guidance
In the press release, EPA Administrator Lee Zeldin says the new guidelines will help keep American operators from losing days in the field or on the road due to faulty DEF systems.
“Failing DEF systems are not an East Coast, West Coast or Heartland issue – it is a nationwide disaster,” Zeldin comments. “I have heard from truck drivers, farmers and many others complaining about DEF and pleading for a fix in all 50 states.”
“Americans are justified in being fed up with failing DEF system issues,” he continues. “EPA understands this is a massive issue and has been doing everything in our statutory power to address this. Farmers and truckers should not be losing billions of dollars because of repair costs or days lost on the job.”
EPA’s new guidance will allow manufacturers to stop inaccurate DEF system failures by removing traditional urea quality sensors and switching to NOx sensors, and the agency affirms approved NOx sensor-based software updates can be installed on existing engines without being treated as illegal tampering under the Clean Air Act.
EPA anticipates the switch will “greatly curb errors that traditional sensor technologies have been prone to and reduce the issues Americans face with inaccurate DEF failures,” according to the agency’s press release.
The agency adds the guidance does not weaken or remove emissions standards, but rather ensures standards are met in a practical, real-world manner.
“EPA remains committed to protecting human health and the environment while ensuring Americans who feed, clothe and power our economy are not burdened by unnecessary system failures,” the agency states.
Longstanding issues
Truckers, motorists and farm equipment operators have long expressed concerns over failure of urea-quality sensors, reporting DEF system failures have triggered unnecessary vehicle derates or shutdowns even when DEF quality was not compromised.
EPA explains, since 2010, most on-road diesel trucks and many types of nonroad equipment including tractors and construction machinery have used selective catalytic reduction systems which inject DEF into exhaust streams to reduce NOx emissions.
In August of 2025, EPA issued new guidance calling on engine and equipment manufacturers to revise DEF system software in existing vehicles and equipment to massively reverse deratements which harm farmers and truckers, according to the agency.
“Prior to EPA’s August 2025 guidance, when DEF ran out or there was an unexpected mechanical failure, systems forced a vehicle to drastically reduce speed or become inoperable,” EPA explains. “In many cases, vehicles were limited to as little as five miles per hour within hours of a DEF-related fault. This caused needless frustration, operational delays and real economic hardship for operators.”
Industry reaction
Multiple trucking and farm advocacy groups have praised the latest DEF guidance, calling it a commonsense solution to issues which have plagued diesel operators and put a damper on productivity for decades.
In a March 27 press release, American Trucking Associations (ATA) Vice President of Energy and Environmental Affairs Patrick Kelly praised the additional flexibility afforded by the updated guidance.
“ATA welcomes EPA’s updated guidance providing additional flexibility on DEF quality monitoring and preventing forced shutdowns,” Kelly says. “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 compounded by widespread parts shortages in recent years. 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.”
“EPA’s decision to provide manufacturers with flexibility to suspend these inducements and eliminate problematic sensors altogether by monitoring a truck’s actual emissions is a pragmatic solution reflecting how these systems perform in the real world,” Kelly continues. “ATA appreciates EPA’s willingness to listen to industry and act. We will continue working with the agency and our industry partners to ensure emissions regulations are both effective and workable for the men and women who keep America’s goods moving.”
U.S. Secretary of Agriculture Brooke Rollins also expressed support for the updated guidance, emphasizing the Trump administration remains committed to serving American farmers and truckers.
In the March 27 EPA press release, Rollins reiterates the updated guidelines will allow engine manufacturers to update their systems to monitor the specific pollutants the EPA is targeting rather than DEF itself, protecting equipment and the environment alike.
“I’m proud to see the EPA responding to the demands of consumers and working to implement real change to solve these issues,” says Rollins. “This powerful step will provide much needed relief, but we will continue to work towards a solution to protect the environment while eliminating damages to farmers and truckers nationwide.”
Grace Skavdahl is the editor of the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net.
