Paccar, Cummins roll out DEF derate software updates
New calibrations raise final DEF-inducement speed from 5 to 25 mph and expand service options for eligible Paccar and Cummins engines without removing emissions-system obligations.
Paccar and Cummins have started deploying engine-software calibrations that change how trucks respond when the diesel exhaust fluid system reports an empty tank, poor fluid quality or a component fault. The central operational change is substantial: under revised federal guidance, the final inducement speed rises from 5 mph to 25 mph, while the time before final inducement for component-related problems is extended. The update preserves the requirement to repair the fault; it gives a driver more practical mobility to reach a safe location or service facility.
Which engines are covered
Cummins says its program can reach more than 1.5 million medium- and heavy-duty engines, including certain model-year 2017 units. Some model-year 2026 production engines already receive the calibration, with timing varying by platform and model year. More 2026 on-highway engines are scheduled to enter production with it from mid-August. Updated off-highway calibrations are planned for model-year 2027 and newer engines. Eligibility covers EPA-certified on-highway engines using selective catalytic reduction; off-highway coverage includes qualifying EPA and EPA/EU SCR engines.
Paccar is offering the software for trucks with MX-11 and MX-13 engines. Trucks manufactured after July 20 are to receive it at the factory, while eligible Kenworth and Peterbilt trucks built after 2018 can be updated through their dealers. The rollout follows Daimler Truck North America, which began updates in February for roughly 330,000 in-service vehicles: DD15-powered model-year 2021-2025 trucks and DD13-powered model-year 2022-2025 trucks. Freightliner and Western Star deployment is continuing through 2026.
What does not change
Selective catalytic reduction still needs DEF to turn nitrogen oxides into nitrogen and water. The fluid remains the standard mixture of 32.5% urea and 67.5% deionized water. Inducements remain compliance safeguards: the engine reduces speed or power to prompt a refill, sensor or component repair, or a regeneration. A 25 mph ceiling is not permission to continue normal dispatch, and operators should not treat a warning as resolved merely because the truck remains mobile.
Impact on operators
The practical benefit is lower exposure to a 5 mph crawl on a shoulder or remote route, along with more time to route equipment to qualified service. That can reduce towing, missed appointments and roadside risk. Benefits will not be uniform, however. Availability depends on engine serial number, model year, application and the manufacturer’s release schedule; shop capacity and calibration time also affect downtime.
Recommended fleet actions
- Build a VIN and engine-serial-number list, then ask the relevant dealer or authorized Cummins location to confirm eligibility and software availability.
- Schedule updates with preventive maintenance rather than waiting for a DEF fault, and record the calibration identifier and completion date.
- Brief drivers that 25 mph is a recovery mode, not an operating target; require immediate reporting of DEF lamps, countdowns and diagnostic messages.
- Keep DEF-quality, sensor, dosing and regeneration troubleshooting procedures intact. Clear codes only after the underlying condition is diagnosed.
- Review roadside and towing plans so dispatchers know the safest approved destination when an inducement begins.
The calibration is therefore an uptime and safety improvement, not an emissions exemption. Fleets that verify each asset, update proactively and preserve disciplined aftertreatment maintenance stand to gain the most.