What is SPN 520365 FMI 5:
SPN 520365 FMI 5 indicates a low current condition in the Diesel Exhaust Fluid (DEF) tank heater circuit. This means the system has detected less current than expected when the DEF heater is active. DEF tank heaters are essential in cold climates to prevent DEF from freezing. A low current fault suggests a break in the circuit, degraded wiring, or a failing heater element.
Why This Happens
- Open circuit or high resistance in the DEF tank heater wiring.
- Faulty or degraded DEF tank heater.
- Corroded or loose electrical connectors.
- Blown fuse or failed relay in the heater control circuit.
- Faulty Aftertreatment Control Module (ACM) not supplying proper voltage.
How to Fix It
- Inspect the DEF tank heater wiring and connectors for damage or corrosion.
- Check fuses and relays in the DEF heater circuit.
- Test the resistance of the DEF tank heater—replace if out of spec.
- Measure voltage and current at the heater connector during operation.
- Replace the DEF tank heater if low current persists despite good wiring.
How to Avoid It Before It Happens
- Perform regular electrical inspections on DEF system wiring and heaters.
- Keep connectors clean, sealed, and moisture-free.
- Use winter-grade DEF and store it properly to reduce freezing risk.
- Replace heaters according to OEM preventive maintenance schedules.
- Monitor heater performance with diagnostics during cold-weather service checks.
- Leverage Clue’s Equipment Tracking to monitor DEF tank heater performance in real-time and ensure it’s functioning properly during cold conditions.
