Reductant System Performance

PACCAR MX code P204F (Reductant System Performance) is a serious-severity diagnostic trouble code set by the engine or aftertreatment control module on MX-11 and MX-13 engines in Kenworth and Peterbilt trucks. It indicates a fault in the aftertreatment system that should be diagnosed with DAVIE4 or PACCAR ESA.

Possibly — short distances may be OK, but ignoring the fault risks progressive damage, regen failure, or derate. Address promptly.

Typical repair runs $500-$4,200. Sensor or wiring repairs are at the low end; injector, DEF pump, DPF, or EGR cooler jobs are at the high end. Dealer-level DAVIE4 software is often required for programming or calibration after parts replacement.

California's heavy-duty OBD regulation requires the system to flag a NOx-converting catalyst when its conversion capability drops enough to push NOx emissions more than 0.4 g/bhp-hr above the applicable standard (Title 13 CCR 1971.1, section (e)(6.2.1)) — so on a 0.2 g/bhp-hr engine the trip point is around 0.6. Catalysts in series may be monitored individually or as a group, which is why a code can point at the aftertreatment as a whole rather than at one brick. This threshold comes from the heavy-duty OBD regulation, which covers 2010 and later heavy-duty engines. If your engine is in a pickup at or under 14,000 lb GVWR it is certified under the separate medium-duty rule instead, so treat this as the design intent rather than your engine's exact limit.