EPA's New Diesel Exhaust Fluid Guidelines Provide Helpful Fix

EPA Issues New Guidance to Prevent Sudden Power Losses Due to Diesel Exhaust Fluid

On August 12, 2025, the U.S. Environmental Protection Agency (EPA) released new guidance to help prevent sudden speed and power losses caused by diesel exhaust fluid (DEF) system faults. The announcement was made by EPA Administrator Lee Zeldin at the Iowa State Fair.

The goal is to reduce unexpected shutdowns for farmers, truck drivers, and equipment operators. EPA says these changes will still meet environmental standards.

Purpose of the New Diesel Exhaust Fluid Guidance

The new recommendations urge engine and equipment manufacturers to update DEF system software in existing vehicles and machinery. This will help prevent sudden “derates.”

A derate happens when a diesel engine quickly reduces power or speed because of a DEF problem.

Currently, if diesel exhaust fluid runs out or a sensor fails, many systems reduce vehicle speed within hours. In some cases, the limit can be as low as five miles per hour. These measures were designed to enforce emissions rules. However, they have also caused costly delays in farming, trucking, and construction.

The EPA says the new approach will give operators more time to fix DEF issues before the loss of power. The guidance also gives manufacturers a clear process for making changes without needing extra EPA approvals.

Changes for Operators Under the DEF Update

Starting with model year 2027, all new on-road diesel trucks will be built to avoid sudden power loss after a diesel exhaust fluid fault or depletion.

For vehicles and equipment already in use, the new guidance allows software updates to be applied to the current fleet. EPA says this will help ensure that solutions reach the field more quickly. Manufacturers will not have to go through extra approval steps before releasing updates.

How Diesel Exhaust Fluid and SCR Technology Work

DEF is made from urea and deionized water. It is used in selective catalytic reduction (SCR) systems.

SCR technology has been common in the United States since 2010. It was introduced to meet the EPA’s Tier 4 emissions standards, which aim to reduce nitrogen oxides (NOx).

The system works in three main steps:

1. Diesel exhaust fluid is sprayed into the engine’s exhaust stream.
2. The fluid reacts with exhaust gases in the SCR catalyst.
3. NOx is converted into nitrogen and water vapor, which are harmless.

Sensors monitor the process to make sure the correct amount of DEF is used. If the fluid is missing, contaminated, or the system malfunctions, the vehicle will eventually derate. This is meant to stop it from operating outside compliance.

Concerns About Diesel Exhaust Fluid Derates

Farmers, truckers, and equipment operators have raised concerns about the current diesel exhaust fluid derate process. Some of the main issues include:

• Unexpected downtime. Power loss can happen during critical work, such as harvesting or hauling.
• High costs. When equipment becomes inoperable, it can require towing or emergency repairs.
• False triggers. A faulty sensor can cause the same severe power limits as a true DEF shortage.

In many rural or remote areas, getting repairs quickly is not possible. This can make delays worse.

Economic Impact of the Diesel Exhaust Fluid Changes

EPA estimates that the change could save family farms about $727 million per year. This comes from preventing lost work hours and avoiding expensive service calls.

The trucking and construction industries are also expected to benefit. The guidance could reduce DEF-related breakdowns and improve reliability for all diesel operators.

The update is not a formal regulation. However, it provides a clear path for manufacturers to update software without added EPA review. The agency hopes this will speed up the release of fixes.

Balancing DEF Compliance and Equipment Reliability

The EPA says protecting air quality and public health remains a priority. The new guidance is designed to keep emissions compliance in place while giving operators more flexibility.

By allowing more operating time before a diesel exhaust fluid derate, the agency expects to cut disruptions without increasing pollution.

Administrator Zeldin said the EPA will continue working with manufacturers, industry groups, and operators to find practical solutions. The agency plans to monitor results to make sure the changes are effective.

Industry Response and Next Steps

Manufacturers are now reviewing the diesel exhaust fluid guidance. They will decide how to roll out updates for both new and existing vehicles.

Operators can expect updates to be made available through dealerships or service centers.

Until then, the EPA advises operators to:

• Check DEF systems regularly.
• Keep DEF supplies on hand.
• Respond to system alerts quickly.

DEF Background

• Since 2010, most heavy-duty diesel engines in the U.S. have used SCR systems with diesel exhaust fluid.
• Nitrogen oxides are linked to smog and respiratory issues.
• On-road trucks, agricultural machinery, and construction equipment all use DEF systems to meet federal emissions rules.

Key Takeaways

• EPA issues guidance to prevent sudden power loss from diesel exhaust fluid system faults.
• Manufacturers urged to update software on existing vehicles and equipment.
• New trucks from model year 2027 must avoid severe power loss after DEF issues.
• The change could save farms an estimated $727 million annually.
• Guidance aims to keep environmental standards while improving reliability.

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