Diesel Exhaust Fluid (DEF) systems have been a common part of modern diesel trucks for years. While these systems help diesel engines meet emissions requirements, DEF-related problems have also become a major headache for truck owners and operators.

Now, the U.S. Environmental Protection Agency (EPA) has taken steps to address one of the biggest sources of those problems: the DEF quality sensor.

What Changed?

On March 27, 2026, the EPA issued new guidance that removes the requirement for diesel equipment manufacturers to use traditional DEF quality sensors, also known as urea quality sensors. Manufacturers can instead use more reliable NOx sensors to monitor the emissions system.

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