US EPA issues revised DEF guidance for diesel vehicles with SCR systems
13 August 2025
The US Environmental Protection Agency (EPA) has issued a revised guidance for diesel vehicles equipped with SCR systems [6523], relaxing the ‘inducement’ requirements—such as vehicle speed and/or torque derates—that must be applied when the vehicle is operated without urea solutions (diesel exhaust fluid, DEF) or under conditions that could otherwise degrade the performance of SCR systems.
The new guidance, announced by the EPA Administrator Lee Zeldin at the Iowa State Fair, will protect American farmers, truckers, and other diesel equipment operators from sudden speed and power losses caused by DEF systems, the EPA said in a press release. The guidance urges engine and equipment manufacturers to revise DEF system software in existing vehicles and equipment to prevent sudden engine shutdowns related to SCR system malfunction, giving operators more time to repair faults without impacting productivity or safety.
“We have heard loud and clear from small businesses across the United States that the current DEF system is unacceptable. It is unacceptable that farmers, truckers, construction workers, and many other small businesses continually experience failures of diesel-powered equipment when they need it most—costing millions of dollars in lost productivity,” said EPA Administrator Zeldin.
Starting with model year 2027, the new EPA requirements state that all new diesel on-road trucks must be engineered to avoid sudden and severe power loss after running out of DEF. To fix the problem for vehicles already in use, the new guidance “will work to ensure that the necessary software changes can be made on the existing fleet,” the EPA said.
SCR systems use on-board diagnostics sensors to detect when DEF runs out—if detected, the engine control module initiates the inducement sequence. This involves an automatic derate that rapidly decreases the vehicle’s performance, such as by drastically limiting the engine torque and vehicle speed. However, the inducement sequence can also be triggered by a failure of various sensors used in the SCR system. In 2021, a sensor error led to widespread failures across all vehicle and engine brands, leading to an industry-wide recall.
The new guidance introduces a three-phase, stepwise inducement approach, including the initial, secondary, and final inducement stages. The new requirements for heavy-duty (HD) trucks are summarized in the following table.
| Category | Timing | Level |
|---|---|---|
| Initial | 650 miles / 10 hours | 15% torque reduction |
| Secondary | 4,200 miles / 80 hours | 30% torque reduction |
| Final | 8,400 miles / 160 hours | 25 mph |
| Prior guidance | 4 hours | 5 mph |
The guidance also specifies new requirements for motor coaches, HD pickups and light-duty (LD) cars, and nonroad equipment (typically farming). Nonroad equipment will now have no performance impact for 36 hours, compared to 4 hours under the prior guidance.
The EPA is not requiring separate approvals for SCR software updates that implement strategies provided in the EPA’s guidance. “This ensures that bureaucratic steps do not delay manufacturers’ ability to put solutions into the field,” the agency said.
The new guidance was praised by diesel engine users, manufacturers, and their trade groups—including the American Trucking Association and the Engine Technology Forum.
Source: US EPA