SPN 655 FMI 6: Meaning and Fix
SPN 655 FMI 6 indicates that the ECM has detected a current above normal or a grounded circuit on the fuel injector for cylinder 5. This fault often appears after a wiring harness chafes against the engine block or during cold-start conditions when injector solenoid resistance drops. Technicians frequently encounter this after a valve cover removal or injector replacement if the connector is not fully seated.
Common Symptoms
- Misfire Cylinder 5: Engine runs rough with a distinct miss on cylinder 5, often accompanied by white smoke.
- Reduced Engine Power: ECM activates torque derate to protect the injector driver, limiting engine output.
- Check Engine Lamp: Red or amber MIL illuminates immediately, with the fault logged as active.
- Hard Starting: Extended cranking time due to incomplete combustion on the affected cylinder.
Probable Causes
- Short to Ground: Injector solenoid wire insulation fails, contacting engine ground or cylinder head.
- Shorted Injector Solenoid: Internal coil winding breakdown causes low resistance, drawing excessive current.
- ECM Driver Failure: Internal short in the ECM injector output stage for cylinder 5, often from overvoltage.
- Connector Corrosion: Moisture ingress at the injector or ECM connector creates a low-resistance path to ground.
Advanced Technical Analysis
The ECM monitors injector current using a low-side driver with a sense resistor. When the current exceeds a calibrated threshold (typically 20-25 A for a peak-hold injector), the driver flags FMI 6. The microcontroller compares the measured current against a dynamic model based on battery voltage and injector temperature. A sustained overcurrent for more than 50 ms latches the fault.
Electrical breakdown analysis reveals that a shorted injector solenoid typically presents resistance below 0.5 Ω, compared to a healthy 0.7–1.2 Ω. The ECM’s debouncing timer prevents false triggering from transient spikes, but a solid short bypasses this filter. Real-world cases show that a chafed wire near the cylinder head gasket is the most common root cause.
Upon detecting FMI 6, the ECM immediately cuts power to the cylinder 5 injector driver and enters a safety fallback mode. This includes disabling fuel delivery to cylinder 5 and reducing overall torque by 30-50% to prevent engine damage. The ECM also logs freeze-frame data including engine speed, load, and coolant temperature for post-event analysis.
Long-term diagnostic strategy involves performing a wiggle test on the harness while monitoring injector current with a lab scope. In workshops, this fault often recurs after a DPF regeneration if the injector coil is heat-damaged. Preventive replacement of the injector and harness pigtail is recommended if resistance is borderline. Always verify ECM firmware for known injector driver updates.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check injector harness for chafing, burns, or pinched wires near cylinder 5.
- Resistance Measurement: Measure injector solenoid resistance at the ECM connector; should be 0.7–1.2 Ω.
- Isolation Test: Disconnect injector and check for continuity to ground; any reading below 10 kΩ indicates a short.
- ECM Swap Test: Swap injector driver channels in software or physically to isolate ECM or injector fault.