US EPA issues new guidance on DEF quality sensors
3 April 2026
The US Environmental Protection Agency (EPA) issued a new guidance for manufacturers of vehicles equipped with selective catalytic reduction (SCR) systems. The guidance relaxes the agency’s requirements in regards to monitoring of urea quality (diesel exhaust fluid, DEF) and removes the urea quality sensor (UQS) requirement for all diesel equipment. This relaxation of policy is intended to address concerns from farmers, truckers, and other diesel equipment operators regarding DEF system failures caused by faulty urea quality sensors.
In February, the EPA requested data from major diesel engine manufacturers on DEF system failures. A preliminary review of the collected data suggests that DEF sensors have high failure rates compared to other SCR system components and are a significant source of warranty claims and SCR-related inducement.
The EPA’s new guidance clarifies that under existing regulations, manufacturers can stop inaccurate DEF system failures by removing traditional urea quality sensors and switching to urea quality detection based on NOx sensors. The agency has affirmed that approved NOx sensor-based software updates can be installed on existing engines without being treated as illegal tampering under the Clean Air Act.
From the technical point of view, SCR systems do not require urea quality monitoring to ensure adequate NOx reduction performance. However, the EPA has designated DEF quality to be an “adjustable parameter” that can affect emissions and—under the EPA’s adjustable parameter regulations—must be monitored. In the SCR guidance documents issued in 2013 and 2014, the EPA concluded that urea quality sensors could be installed in new highway vehicles and nonroad equipment to meet the EPA’s adjustable parameter regulations.
While the EPA had not disallowed the use of alternative urea quality detection methods, such as methods based on NOx sensors, manufacturers had to demonstrate the effectiveness of the alternative approaches. Since 2016/2017, only one diesel engine manufacturer has continued to demonstrate compliance with the adjustable parameter requirements using NOx sensors. All other manufacturers switched to the recommended UQS methodology because of the UQS’s ability to directly measure DEF quality.
In the new guidance, the EPA encourages manufacturers to implement alternative methods of meeting the adjustable parameter provisions, including the use of NOx sensors. Further, the EPA is providing an illustrative example of how a manufacturer may make the necessary demonstrations to certify NOx sensor-based systems. The EPA has also stated that software updates to implement the new guidance may be installed in existing diesel engines in place of, or to augment, existing UQS-based systems.
Under the new guidance, the EPA will provide further flexibilities in approving non-UQS strategies to monitor urea quality. For instance, while monitoring strategies are generally expected to make determinations within one hour, the EPA will allow the system to continue to monitor beyond the hour and not to automatically determine there is a fault. The EPA is also willing to approve monitoring strategies that operate only after a DEF tank refill, within a prescribed temperature window, or within certain engine speed and load conditions that support accurate determination of DEF quality.
Source: US EPA