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SPN 3031 FMI 4

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

What the SPN covers: Temperature of the DEF in the tank, read by a sensor in the tank header. It controls tank heating and tells the computer whether the fluid can be pumped. More on SPN 3031.

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

A bad reading can block DEF heating or dosing and lead to inducement derates, especially in cold weather.

Likely causes

1. Wiring or connector fault in this circuit (most common for electrical FMIs)
2. Failed temperature sensor in the tank header (often combined with level/quality)
3. DEF-corroded connector or wiring
4. Tank heater fault making the reading implausible
5. Network message missing from a smart tank sensor

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. Compare the reading with ambient temperature after a cold soak
4. Inspect the header connector for white crust and corrosion
5. Check for related heater, level and quality codes
6. Check for network codes if the sensor reports over CAN

FAQ