2019 International LT - Cummins X15 - SPN 3363 FMI 3 DEF Tank Heater High

Question

Working on a 2019 LT with the X15. It keeps throwing SPN 3363 FMI 3, which points to the DEF tank heater. We are in a cold climate and if the DEF freezes, the truck won't start or goes into derate.

I've checked the resistance on the tank heater and it seems within spec, but the code persists. Does this usually mean the coolant valve is stuck or is it an electrical short in the harness?

GearheadGus · Accepted Answer

✅ Confirmed fix: The wiring harness rubbing against the frame rail near the transition from the cab to the frame had three chafed wires. One was the 5V reference for the DEF tank, causing the 'voltage high' signal. Repaired the wires and added protective looming. The code cleared and hasn't returned.

FleetMech_Ed · Answer

If you swapped the header and it didn't fix it, look at the DEF coolant valve located on the frame rail. If the solenoid on the valve shorts out, it can throw that heater code because the ECM isn't seeing the right feedback.

2019 International LT - Cummins X15 - SPN 3363 FMI 3 DEF Tank Heater High

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