EPA Extends DEF Derate Grace Period to 650 Miles, 25 mph Floor

EPA guidance issued in 2026 extends the grace period before diesel exhaust fluid (DEF) system faults trigger engine derates, giving fleets up to 650 miles of warning-only operation and raising the final speed floor from 5 mph to 25 mph. Daimler Truck North America (DTNA) is rolling out software updates to implement the changes on existing trucks.

How long can a truck run after a DEF fault before derate kicks in?

After a DEF system fault is detected, only a warning light appears for the first 650 miles or 10 hours of operation. No performance impact during that window. A driver who catches the fault at the start of a shift has at least one full workday to reach a repair facility without losing power.

At 650 miles or 10 hours, the engine derates by 15% under the EPA guidance, but speed is not limited. The truck can operate normally for up to 4,200 miles, roughly two work weeks, before further restrictions apply.

Only after 10,500 miles, about four work weeks from the initial fault, does the vehicle see a speed derate down to 25 mph. That floor previously sat at 5 mph, a threshold that left trucks stranded on highways and forced expensive tows. The 25 mph floor lets a driver limp to a shop under their own power.

What changed in EPA's DEF enforcement policy?

EPA announced in early 2026 that it was re-examining DEF sensor failures and the derate schedules OEMs had programmed into diesel engines. The agency published guidance in March 2026 targeting DEF sensor failures behind truck derates, then followed with the extended grace-period framework that DTNA and other manufacturers are now implementing through software.

The Justice Department separately pulled back on criminal prosecution of diesel emissions deletes in January 2026, signaling a broader shift in federal enforcement priorities around aftertreatment systems. EPA's guidance on derates followed that policy change.

Starting with model year 2027, all new diesel on-road trucks must be engineered to avoid sudden and severe power loss after running out of DEF. That requirement applies to new production only. For vehicles already in use, EPA's guidance allowed necessary software changes to be made to the existing fleet, which is what DTNA and other OEMs are deploying now.

What fleets need to do with the software update

DTNA's software update is available through authorized service centers. Fleets running Freightliner or Western Star trucks with selective catalytic reduction (SCR) systems should contact their dealer to schedule the reflash. The update changes only the derate timing and thresholds programmed into the engine control module (ECM). It does not alter emissions hardware or require new parts.

The 650-mile warning window and 25 mph speed floor apply only after the software update is installed. Trucks still running the old derate schedule will hit the 5 mph floor at the original mileage threshold. Fleets that delay the update risk a roadside breakdown that could have been avoided with an extra 20 mph of limp-home capability.

The extended grace period does not eliminate the need to repair DEF faults. A truck that reaches the 10,500-mile threshold and derates to 25 mph will remain at that speed until the fault is cleared. The update buys time to schedule a repair during normal downtime, not permission to run indefinitely with a malfunctioning aftertreatment system.

DEF system compliance under the new derate schedule

The EPA guidance and DTNA software update do not change a fleet's obligation to maintain functional SCR systems. Diesel engines certified under EPA emissions standards must operate with DEF injection and SCR catalyst systems working as designed. A truck that runs out of DEF or develops a sensor fault is still out of compliance once the derate takes effect.

What changed is the enforcement mechanism. The old 5 mph derate was intended to force immediate repair but often left trucks stranded in unsafe locations. The new schedule gives fleets a realistic window to diagnose the fault, order parts, and schedule shop time without losing a truck in the middle of a load.

Fleets should treat the 650-mile warning light as a hard deadline to begin troubleshooting. Waiting until the 15% derate kicks in wastes half the available grace period. The two-week window at 15% derate is meant for parts delivery and shop scheduling, not for continuing normal dispatch.

What the 2027 model-year requirement means for new truck specs

EPA's mandate that 2027 and later trucks avoid sudden power loss after DEF depletion will likely push OEMs toward larger DEF tanks, more accurate level sensors, and earlier low-level warnings in the dash. Fleets ordering 2027 model-year trucks should ask dealers what changes the manufacturer made to meet the EPA requirement.

The 2027 rule does not grandfather existing trucks. A 2026 or earlier truck will not be required to meet the 2027 standard unless the manufacturer issues a voluntary recall or the fleet chooses to install updated software. DTNA's current software update is voluntary and applies the extended derate schedule to trucks already in service, but it is not a recall and does not carry a compliance deadline.

Fleets running mixed-age equipment will need to track which trucks have the updated derate schedule and which are still on the old 5 mph floor. That matters for dispatch planning. A truck with the old software that throws a DEF fault 200 miles from the nearest dealer is a tow bill waiting to happen.

How this affects CSA scores and roadside inspections

A truck operating under derate because of a DEF fault is not automatically cited during a roadside inspection, but an inspector who finds a malfunctioning emissions system can place the vehicle out of service under CVSA criteria. The extended grace period does not create a safe harbor from OOS orders.

Fleets should document DEF faults and repair attempts in their maintenance records. If a truck is stopped while operating at 15% derate, the driver should be able to show the inspector that the fault was detected within the grace period and that a repair appointment is scheduled. That documentation will not prevent an OOS order if the inspector determines the emissions system is non-functional, but it demonstrates the fleet is not ignoring the fault.

The 25 mph speed floor itself is not a CSA violation, but a truck crawling at 25 mph on a highway creates a traffic hazard that can draw law enforcement attention. Drivers should use the grace period to reach a repair facility, not to continue hauling loads at reduced speed.

What to update in your maintenance logs this week

Fleets should add a line item to their preventive maintenance checklist for DEF system health. Check DEF level at every fuel stop. Inspect the DEF tank filler neck and cap for contamination. Test the DEF quality with a refractometer if the truck has been sitting idle or if the DEF is more than a year old.

When a DEF fault warning appears, log the mileage, the fault code, and the date. That creates a paper trail showing the fleet responded within the 650-mile window. If the truck reaches the 15% derate threshold, log that mileage as well and document the repair appointment.

Fleets using Carrier Atlas's electronic logging device review should confirm their ELD captures engine fault codes and can export them for maintenance records. Some ELD platforms flag DEF faults automatically and send alerts to dispatch. That feature is worth enabling if your ELD supports it, because it gives the fleet a heads-up before the driver notices the warning light.