US EPA requests DEF system failure data from manufacturers
5 February 2026
The US Environmental Protection Agency (EPA) announced it has requested information from major diesel engine manufacturers on critical data from Diesel Exhaust Fluid (DEF) system failures. The data collected will allow EPA to independently evaluate ongoing system failures and help inform next steps the agency takes in 2026 pertaining to DEF.
The action is intended to address “widespread concerns” from farmers, truckers, motor coach operators, and other diesel equipment operators regarding DEF system failures. As a follow up to the guidance issued in August 2025, relaxing the inducement requirements for diesel engines with SCR systems that “were harming farmers and truckers”, the EPA said it is pursuing rulemaking opportunities to provide further relief for diesel equipment owners.
Under the Clean Air Act (CAA), the EPA is authorized to require manufacturers to provide information needed to assess whether emission control systems are functioning properly. The EPA said it is demanding information from the top 14 on-road and nonroad manufacturers that account for over 80% of all products used in DEF systems. Specifically, the agency is requiring data on warranty claims, failure rates, and repair information for MY 2016, 2019, and 2023 emission control products to determine whether ongoing DEF system failures are related to a specific generation of products. Manufacturers were given 30 days to provide the requested information.
“As I traveled to all 50 states during my first year as EPA Administrator, I heard from truck drivers, farmers, and many others rightly complaining about DEF and pleading for a fix. EPA understands this is a massive issue, which is why we have already established commonsense guidance for manufacturers to update DEF systems,” said EPA Administrator Zeldin. “Today, we are furthering that work and demanding detailed data to hold manufacturers accountable for the continued system failures.”
Most on-road diesel trucks and many types of nonroad equipment (such as tractors and construction machinery) use SCR systems that inject urea solutions, a.k.a. DEF, into exhaust streams to reduce NOx emissions. Diesel engines with SCR systems are equipped with driver/operator inducement strategies, such as engine deratement or shutdown, to prevent engine operation without DEF. The EPA’s guidance of August 2025 significantly reversed deratements, and called on manufacturers to revise DEF system software in existing fleets to prevent sudden engine shutdowns and give operators more time to repair faults. With the request for SCR system failure data, the EPA intends to address any potential insufficiencies with system parts to better advance future rulemaking and reduce system failures.
In addition to its August 2025 guidance, the EPA stated that starting with MY 2027, all new diesel on-road trucks must be engineered to avoid sudden and severe power loss after running out of DEF. The agency said that its ongoing work on a proposal for the reconsideration of the 2022 Heavy-Duty Engine and Vehicle NOx rule includes an assessment whether derates may no longer be necessary for compliance.
The EPA has also issued a guidance to manufacturers, clarifying that under the CAA, equipment owners have the right to repair their farm and other nonroad diesel equipment. Specifically, a temporary disablement of the SCR inducement strategy is not prohibited under the CAA when it is for the purpose of repair or maintenance of a nonroad engine. This guidance will allow farmers to fix their own equipment, the EPA said, while the agency continues its work on DEF.
Source: US EPA