Turbocharger Overboost

PACCAR MX code P0234 (Turbocharger Overboost) 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 air 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 $300-$3,500. 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 sets the underboost trip point where a decrease from commanded or expected boost would push NMHC, CO or NOx past 2.0 times the applicable standards, or PM past the standard plus 0.02 g/bhp-hr (Title 13 CCR 1971.1, section (e)(4.2.1)). As with EGR, it must also flag the system reaching its control limits without achieving commanded boost — worth knowing before condemning a turbo that is physically moving. 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.