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Donald J. Trump @realDonaldTrump - EPA
United States
Environmental Protection Agency
Fact Sheet: Diesel Exhaust Fluid (DEF) Sensor Guidance Letter
As EPA Administrator Lee Zeldin traveled to all 50 states during his first year as Administrator, he heard from truck drivers, farmers, and many others rightly complaining about DEF and pleading for a fix.
EPA's March 27, 2026, DEF Sensor Guidance implements another part of Administrator Zeldin's plan to help keep American operators from losing days in the field or on the road because of faulty DEF systems.
The guidance removes DEF sensor requirements and allows Americans the option to use more reliable sensors, saving operators over $13 billion annually, not to mention countless hours of time and productivity of avoided breakdowns.
DEF Actions Taken by Trump EPA
* In response to concerns, EPA issued guidance in August 2025 calling on engine and equipment manufacturers to revise DEF system software in existing vehicles and equipment to massively reverse deratements that were harming farmers, truckers, and others.
* EPA will be issuing a new deregulatory proposal this spring that will completely remove all DEF deratements for new vehicles and engines. With these changes, sudden speed losses, which compromise safety and productivity, will be an issue of the past.
* Building on that effort, EPA took further action in February 2026 by issuing an information demand letter to the top 14 on-road and nonroad manufacturers that account for over 80 percent of all products used in DEF systems to better understand the scope and causes of DEF system failures.
* Additionally, in February 2026, EPA reaffirmed American farmers and equipment owners' right to repair their farm and other nonroad diesel equipment, including faulty DEF systems, in the field.
EPA's clarifying guidance made clear manufacturers could no longer use the Clean Air Act to justify
limiting access to repair tools or software.
* On March 27, 2026, based on preliminary findings from the demanded data, EPA issued additional guidance making clear that under existing regulations, manufacturers can stop inaccurate DEF system failures by removing traditional emission sensors, known as Urea Quality Sensors, and switching to nitrous oxide (NOx) sensors.
* This guidance will provide immediate relief and according to the U.S. Small Business Administration provide $13.79 billion a year of savings to Americans.
What is a DEF Inducement?
* Selective Catalytic Reduction (SCR) systems use on-board diagnostics sensors to monitor DEF levels and quality. When a problem is detected, the engine control system automatically initiates warnings and reduces vehicle performance. In some cases, within a few hours the system can significantly limit speed, which is commonly referred to as a derate or inducement.
* These inducements were originally implemented to ensure compliance with EPA's 2010 on-highway NOx standards and Tier 4 emissions standards for nonroad engines.
* However, when sensors fail or malfunction, they can trigger an automatic inducement even if there is sufficient DEF in the system and no actual emissions issues.
* In 2021, a DEF sensor error led to widespread failures across all vehicle and engine brands, leading to an industry-wide recall.
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