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SPN 3363 FMI 4

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

What the SPN covers: The coolant valve or electric element that thaws and warms DEF in the tank so it can be pumped in cold weather. More on SPN 3363.

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

In warm weather you can usually keep running; in freezing weather DEF may not thaw and derates can follow.

Likely causes

1. Wiring or connector fault in this circuit (most common for electrical FMIs)
2. Stuck or failed coolant heater valve
3. Open electric heater element in the tank header
4. Wiring or connector fault at the tank header
5. Header replaced without reseating seals and connectors

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. Command the heater with a scan tool and feel/measure the coolant line warming
4. Measure heater element resistance against spec
5. Inspect header connectors and O-rings, especially after recent tank work
6. Check DEF tank temperature reading for plausibility

FAQ