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SPN 3363 FMI 7

Aftertreatment 1 DEF Tank Heater — Mechanical system not responding or out of adjustment.
The controller commanded something to move or change, and the result did not match the command.

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 7 usually points to: Stuck or sticking valve or actuator, binding linkage, carbon buildup, or a component worn out of adjustment. More on FMI 7.

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. Stuck or failed coolant heater valve
  2. Open electric heater element in the tank header
  3. Wiring or connector fault at the tank header
  4. Header replaced without reseating seals and connectors

Checks & fixes (easiest first)

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

FAQ

What does SPN 3363 FMI 7 mean?

Aftertreatment 1 DEF Tank Heater — mechanical system not responding or out of adjustment. The controller commanded something to move or change, and the result did not match the command.

Is SPN 3363 FMI 7 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 7?

Command the heater with a scan tool and feel/measure the coolant line warming; Measure heater element resistance against spec; Inspect header connectors and O-rings, especially after recent tank work.