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SPN 3363 FMI 18

Aftertreatment 1 DEF Tank Heater — Value too low (moderately severe).
The reading is below normal at a middle warning level — more serious than FMI 17, less than FMI 1.

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 18 usually points to: A developing under-condition that often precedes a derate. More on FMI 18.

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 18 mean?

Aftertreatment 1 DEF Tank Heater — value too low (moderately severe). The reading is below normal at a middle warning level — more serious than FMI 17, less than FMI 1.

Is SPN 3363 FMI 18 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 18?

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.