Diagnostic Overview
SPN 3251 FMI 6 is an aftertreatment DPF pressure-related fault on the PACCAR MX platform. It usually means the ECM is seeing a diesel particulate filter pressure, restriction, or differential-pressure signal that is outside the expected range. This can point to a soot-loaded or ash-loaded DPF, but it can also be caused by sensor tubing, wiring, or a pressure sensor problem. On PACCAR MX engines, confirm the exact definition with DAVIE4, the engine serial number, emissions year, and current calibration because code meanings and troubleshooting trees can vary.
Likely Causes
Soot-loaded DPF; ash-loaded DPF; failed DPF differential pressure or outlet pressure sensor; plugged, cracked, or wet sensor tubes; exhaust restriction; excessive soot from EGR, turbo, injector, or air-handling issues; damaged wiring or connector corrosion.
What To Check First
Check all active and inactive aftertreatment faults first, inspect the DPF pressure tubes and sensor connector, compare live DPF pressure readings with key-on/engine-off and running values, review regen history, and determine whether a forced regen, filter cleaning, or sensor/circuit repair is required.