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EPA Moves to Address DEF System Failures
Updated Guidance Changes Sensor Requirements With Aim of Reducing Shutdowns, Costly Repairs
Key Takeaways:
- EPA approved guidance allowing diesel manufacturers to replace urea quality sensors with NOx sensors in DEF systems.
- The agency said faulty DEF sensors are a significant source of warranty claims and vehicle shutdowns.
- EPA projected billions in annual savings and said approved software updates will not be treated as illegal tampering.
The has approved a change regarding for diesel exhaust fluid systems in response to complaints from truck owners frustrated with systems failures and vehicle shutdowns.
“Americans are justified in being fed up with failing DEF system issues,” EPA Administrator Lee Zeldin said in a March 27 news release. “EPA understands this is a massive issue and has been doing everything in our statutory power to address this.”
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Under the agency’s updated guidance, manufacturers will be permitted to replace Urea Quality Sensors with nitrogen oxide sensors. The guidance also said that approved NOx sensor-based software updates can be installed on existing engines without being treated as illegal tampering under the Clean Air Act. This, the agency said, aligns with EPA’s February 2026 Right to Repair clarification guidance, which “removed a major barrier keeping farmers from fixing their faulty DEF systems in the field.”
The agency added, “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.”
The move follows EPA’s Feb. 3 request from manufacturers for data on DEF system failures. Thus far, 11 of the 14 manufacturers contacted — representing 80% of all products used in DEF systems — have responded, EPA said.
DEF Sensor Failures
EPA took action on the sensor requirement after a preliminary review of the warranty data pointed to DEF sensor failures as a significant cause of warranty claims and DEF-related inducements.
“In less than a month, EPA has turned around preliminary findings to issue today’s guidance demonstrating Administrator Zeldin’s commitment to fixing this issue,” the news release said. “The preliminary review of the warranty data suggests that DEF sensor failures are a significant source of warranty claims and DEF-related inducement. Farmers and truck drivers should not have their vehicles stop working because a sensor isn’t working properly. EPA is taking immediate action on this new information.”
DEF is used in selective catalytic reduction emissions systems found in many trucks. In most systems, a truck will “de-rate” — or slow down — if it runs low on DEF. In some circumstances, the truck will slow to a crawl and fail to restart once it is shut down after running out of DEF.
EPA said, “It is unacceptable that the sudden speed losses and shutdowns caused by DEF system failures are compromising safety and productivity.”
Industry Response
Zeldin said DEF-related issues have led to a national outcry from truckers, farmers, motor coach operators and other diesel equipment operators.
The new guidance aims to save billions of dollars in repairs and lost productivity. According to the U.S. Small Business Administration, farmers alone will save $4.4 billion a year. More broadly, it projected $13.79 billion in industrywide annual savings.
Zeldin said, “Farmers and truckers should not be losing billions of dollars because of repair costs or days lost on the job.”
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SBA Administrator said, “I applaud Administrator Zeldin for his leadership on this issue, and I look forward to our continued collaboration to cut red tape for small businesses across the U.S. food supply chain.”
American Trucking Associations backed the new guidance.
“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,” said Patrick Kelly, ATA vice president of Energy and Environmental Affairs. “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.
Kelly lauded EPA for taking action.
“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,” he said. “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.”
