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SPN 3363 FMI 5

Aftertreatment 1 DEF Tank Heater — Current below normal or open circuit.
Less current is flowing in an output or sensor circuit than expected, which usually means the circuit is open.

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 5 usually points to: Broken wire, unplugged connector, burned-out coil or solenoid, or a backed-out terminal. More on FMI 5.

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

What does SPN 3363 FMI 5 mean?

Aftertreatment 1 DEF Tank Heater — current below normal or open circuit. Less current is flowing in an output or sensor circuit than expected, which usually means the circuit is open.

Is SPN 3363 FMI 5 serious?

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

What should I check first for SPN 3363 FMI 5?

Inspect the connector and harness for this circuit: corrosion, water, chafing, backed-out pins; Check the reference voltage, ground and signal at the component per OEM pin-out; Command the heater with a scan tool and feel/measure the coolant line warming.