SCR or NOx check engine light: how many miles before the truck derates
Under EPA guidance from Aug. 11, 2025, a heavy truck with updated software shows only a warning during the first 650 miles or 10 hours following an SCR fault, then loses 15% of torque, and drops to 25 mph only after 8,400 miles or 160 hours. Trucks on older software can still slow to 5 mph within hours.
Why it matters: On trucks with updated software, an SCR or NOx light starts a warning period of 650 miles or 10 hours before any power loss, but older calibrations can still cut speed to 5 mph within hours. Find out which software the engine runs before planning the run to the shop.