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PACCAR Raises DEF Limp-Mode Speed to 25 MPH in Major Emissions Enforcement Shift

PACCAR is implementing software updates for MX-11 and MX-13 engines that change how diesel exhaust fluid faults impact truck operations. This marks a significant shift in regulatory enforcement. The update raises the maximum limp-mode speed from 5 mph to 25 mph, allowing drivers to reach repair facilities rather than becoming stranded on roadways. Additionally, the manufacturer extended the timeline before severe derates occur from 4 hours to 160 hours, giving operators substantially more time to diagnose and address component or fluid-quality issues. Trucks manufactured after July 20, 2026 will receive the updated software at the factory, while Kenworth and Peterbilt dealerships can retrofit vehicles built after 2018. PACCAR's changes reflect broader EPA regulatory adjustments implemented since mid-2025 to ease restrictions following sustained industry complaints.