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SPN 3363 FMI 5 — DEF tank heater — open circuit
SeriousAftertreatmentPower derate likely — plan to stop soon
Applies to: Detroit · Paccar · Sae · Volvo
DEF tank heater — open circuit.
Don't risk it — get a diesel tech on the truck.
Your SPN/FMI rides along, so nearby mechanics see the fault before they quote you.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Open heater element
- Open drive wire
- Corroded connector
- Open wire
- Connector corrosion
- Open heater
- Wiring
First moves (roadside)
- Ohm the heater element
- Measure drive voltage at key-on with cold DEF
- Ohm the heater (check spec)
- Check harness
- Ohm heater
- Check wiring
FAQ
What does SPN 3363 FMI 5 mean?
DEF tank heater — open circuit.
How serious is SPN 3363 FMI 5?
Serious — Power derate likely — plan to stop soon.
What's the most likely cause of SPN 3363 FMI 5?
The most common cause is: Open heater element. Other possibilities include Open drive wire; Corroded connector; Open wire; Connector corrosion; Open heater; Wiring.
Can I keep driving with SPN 3363 FMI 5?
Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Throwing this code right now?
Find a mechanic or related guides
General diagnostic guidance for heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.