The U.S. Environmental Protection Agency on Friday said it is dropping the requirement for diesel exhaust fluid (DEF) sensors as part of a truck's aftertreatment system in favor of monitoring actual emissions, and catalyst efficiency.
Earlier this year, EPA sought data on DEF system failures from the manufacturers that account for over 80% of all products used in DEF systems. Thus far, the agency said it has received data from 11 of the 14 manufacturers, and less than a month later, the preliminary findings helped inform this new guidance.
“Failing DEF systems are not an East Coast or West Coast or Heartland issue; it is a nationwide disaster," said EPA Administrator Lee Zeldin. "I have heard from truck drivers, farmers, and many others complaining about DEF and pleading for a fix in all 50 states... Farmers and truckers should not be losing billions of dollars because of repair costs or days lost on the job.”
The EPA’s updated guidance, announced Friday, allows truck manufacturers to bypass sensors that monitor the quality of DEF. Under previous regulations, if these sensors detected poor fluid quality—or if the sensor itself malfunctioned—the vehicle’s engine would derate, limiting speed or forcing a full shutdown to ensure emissions compliance.
The EPA clarified that manufacturers are not required to use Urea Quality Sensors (UQS) and are encouraged to use alternative methods, such as nitrogen oxide (NOx) sensors, to meet adjustable parameter regulations.
The American Trucking Associations (ATA) praised the shift, noting that the sensors frequently sidelined otherwise safe, compliant trucks due to reliability issues rather than actual environmental hazards.
"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," said Patrick Kelly, ATA vice president of energy and environmental affairs.
The Truck and Engine Manufacturers Association (EMA) identified UQS as having some of the highest failure rates among Selective Catalytic Reduction (SCR) components. The EPA's own data supports that these failures are a significant source of warranty claims.
The issue became a focal point in recent years as global parts shortages made it difficult for fleet owners to find replacement sensors, leaving functional trucks sitting idle for weeks.
Under the new approach, manufacturers can monitor a truck’s actual emissions output rather than relying on the specific DEF quality sensor, allowing the vehicle to remain fully functional as long as it meets environmental standards.
The EPA considers DEF quality an adjustable parameter because operators could intentionally or accidentally dilute the fluid, leading to higher NOx emissions. While DEF sensors were previously favored for the speed in which they can detect dilution, the EPA, in its new guidance, now acknowledges that NOx sensors can provide an adequate restraint on adjustability if the range of non-compliant operation is small enough that operators are unlikely to target it.
"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 that reflects how these systems perform in the real world," Kelly said.
Manufacturers may install software updates on existing diesel products to replace or augment UQS-based systems with alternative technologies. The EPA explicitly stated that these updates will not be considered tampering under the Clean Air Act.
