SPN 3251 FMI 2 — DPF Differential Pressure — Data Erratic
The ECM sees an implausible or erratic reading from the diesel particulate filter (DPF) differential-pressure sensor. The truck can't trust soot-load data, so it may block regens and stage an engine derate to protect the aftertreatment system.
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How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Plugged, kinked, or soot-fouled DPF pressure sensor lines/tubes
- Failed or drifting DPF differential-pressure sensor
- An actually overloaded/plugged DPF skewing pressure readings
- Wiring/connector corrosion or chafe to the pressure sensor
First moves (roadside)
- Read full freeze-frame data — note soot load %, miles since last regen, and any companion SCR/DEF codes.
- Inspect the two pressure-sensor hoses for soot packing, kinks, or melt near the exhaust.
- If soot load is high and a parked regen is allowed, attempt a forced/parked regen in a safe spot.
- Check the sensor connector for corrosion or a backed-out pin before condemning the sensor.
Common misdiagnoses
- Replacing the DPF when only the pressure tubes were plugged with soot.
- Condemning the sensor when the real issue is a fully loaded DPF that needs a shop bake/clean.
- Clearing the code without a regen — it returns within a few miles.
FAQ
The ECM sees an implausible or erratic reading from the diesel particulate filter (DPF) differential-pressure sensor. The truck can't trust soot-load data, so it may block regens and stage an engine derate to protect the aftertreatment system.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Plugged, kinked, or soot-fouled DPF pressure sensor lines/tubes. Other possibilities include Failed or drifting DPF differential-pressure sensor; An actually overloaded/plugged DPF skewing pressure readings; Wiring/connector corrosion or chafe to the pressure sensor.
Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Find a mechanic or related guides
General diagnostic guidance for heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.