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SPN 3251 FMI 4

Aftertreatment 1 DPF Differential Pressure — Voltage below normal or shorted low.
The sensor circuit voltage is lower than it should ever be, usually near zero.

What the SPN covers: Pressure difference across the diesel particulate filter — how hard the exhaust is working to push through it. More on SPN 3251.

What FMI 4 usually points to: Signal wire shorted to ground, a lost 5 V reference, or an open signal wire on a pulled-down circuit. More on FMI 4.

Severity

MEDIUM — fix soon

High readings mean a loaded or plugged DPF. Regenerate or service soon to avoid a severe derate.

Likely causes

1. Wiring or connector fault in this circuit (most common for electrical FMIs)
2. DPF loaded with soot or ash
3. Cracked or melted sensor hoses/tubes
4. Faulty differential pressure sensor
5. Failed regeneration due to other faults

Checks & fixes (easiest first)

1. Inspect the connector and harness for this circuit: corrosion, water, chafing, backed-out pins
2. Check the reference voltage, ground and signal at the component per OEM pin-out
3. Inspect the sensor tubes for cracks, plugging or reversed lines
4. Read soot load and perform a parked regen if allowed
5. Check the sensor key-on, engine-off reading (should be near zero)
6. Have the DPF cleaned or replaced if ash-loaded

FAQ