Diesel Exhaust Fluid
- Overview of Action
- Background on Diesel Exhaust Fluid
- What is EPA doing?
- What has EPA Already Done to Address these Concerns?
- What Does EPA Require for Vehicles?
- What Could Manufacturers be Doing to Alleviate these Issues?
- Documents and Additional Resources
Overview of Action
The Environmental Protection Agency is taking steps to address widespread concerns from farmers, truckers, and other diesel vehicle operators about a loss of speed and power, or engine derates, caused by selective catalytic reduction systems that use diesel exhaust fluid. Specifically, EPA is issuing guidance to manufacturers for how they may modify DEF systems on new and existing diesel vehicles and nonroad equipment to reduce derates that can limit a vehicles performance to nearly inoperable levels (as low as five miles per hour). EPA is urging engine and equipment manufacturers to follow this guidance and develop software updates that align with the Agency’s recommendations.
Background on Diesel Exhaust Fluid
Since 2010, nearly all on-road diesel trucks and many types of nonroad equipment (such as tractors and construction machinery) have used DEF as part of selective catalytic reduction systems to reduce emissions of nitrogen oxides, a major contributor to air quality nonattainment. These systems can force a vehicle to drastically reduce speed or become inoperable if DEF runs out or if system sensors fail.
Although this derate strategy was intended to ensure compliance with EPA’s Tier 4 Emissions Standards (on-highway, nonroad), it has caused needless frustration, operational delays, and real economic hardship for countless farmers, truckers, and equipment operators.
Despite work over the past 15 years to make DEF broadly available at truck stops and fueling stations, challenges remain.
What is EPA doing?
To address the derating problem for older vehicles still in use, EPA has developed new guidance in collaboration with the manufacturers to ensure that the necessary software changes can be made on the existing fleet. In addition to providing certainty to manufacturers regarding how EPA wants them to solve this issue, the Agency has also assured manufacturers that it will not impose additional reporting requirements for these changes to ensure that bureaucracy will not delay the manufacturers’ ability to deploy fixes. Specifically, EPA is not requiring separate approvals beyond that provided in the new guidance.
Why is EPA Taking this Action Now?
The Trump EPA is taking this action in response to ongoing feedback from operators, farmers, and industry stakeholders about real world impacts of diesel engine derates. By issuing guidance and partnering with manufactures to act, we are working to ensure emissions compliance while minimizing disruptions to safety, transportation, and commerce.
What has EPA Already Done to Address these Concerns?
In 2014, EPA issued a Final Rule for Emergency Vehicles which included Selective Catalytic Reduction Maintenance and Regulatory Flexibility for Nonroad Equipment. This rule consisted of three parts.
First, minimum maintenance intervals for DEF replenishment when using SCR technologies. Second, provisions allowing nonroad engine manufacturers to give operators temporary relief from emission controls while in emergency situations (such as those where operation of a nonroad engine or equipment is needed to protect human life). Third, minor revisions to the direct final rule for emergency vehicles that became effective August 7, 2012.
In 2022, EPA finalized a revised inducement strategy, starting with model year 2027 heavy-duty on-highway vehicles and engines. This update was made in direct response to user concerns and included several improvements to serviceability. For example, requiring manufacturers to provide more diagnostic data to independent repair technicians and owners; mandating disclosure of part numbers for critical components (e.g., filters and sensors); and requiring clearer information for users on how emissions systems operate.
What Does EPA Require for Vehicles?
EPA requires vehicles using DEF to meet specific emissions standards, particularly for NOx. These requirements include:
- Emissions Limit Requirements: Vehicles must effectively meet EPA Tier 2, Tier 4, or 2010 emissions standards, depending on the vehicle type and model year.
- On-Board Diagnostics: Vehicles must have OBD to monitor the SCR system to detect low-DEF levels, system failures, and alert the driver via a warning light.
- Tamper Prevention System: The system must be designed to prevent tampering or bypassing DEF usage.
By law, EPA cannot mandate field fixes for in-use vehicles and nonroad equipment, which is why the Agency is issuing voluntary guidance to manufacturers on system modifications to reduce derates.
What Could Manufacturers be Doing to Alleviate these Issues?
EPA is urging engine and equipment manufacturers to follow EPA’s direction and provide software updates for diesel trucks and equipment meeting these new guidelines.
This includes developing software updates for in-use vehicles that extend the time available to resolve certain SCR-related issues that trigger inducements (e.g., DEF quality or tampering). Further, it includes working closely with their customers to identify which engines should be prioritized for new software first and ensuring that customers are aware of this potential update.
Documents and Additional Resources
- Fact Sheet: Diesel Exhaust Fluid Guidance (pdf)
- Revised Guidance for Light Duty Vehicles, Heavy-Duty Diesel Engines and Nonroad Compression-Ignition Engines Using Selective Catalyst Reduction Technologies (pdf)
- EPA Press Release: At Iowa State Fair, EPA Administrator Zeldin Announces Diesel Exhaust Fluid Fix (August 12, 2025)