SPN 655 FMI 4: Meaning and Fix
SPN 655 FMI 4 indicates cylinder 5 fuel injector experiencing voltage below normal or short-to-ground condition. This fault commonly appears after engine bay water ingress or during cold weather startups when moisture penetrates injector connectors. The ECM detects insufficient voltage feedback from the injector solenoid circuit, triggering immediate fuel delivery compensation and potential power derate to protect the combustion system from uneven firing patterns.
Common Symptoms
- Engine Roughness: Noticeable vibration and irregular idle patterns due to cylinder 5 misfiring or reduced fuel delivery.
- Power Loss: Reduced engine performance and acceleration response from compromised fuel injection timing on cylinder 5.
- Increased Emissions: Higher exhaust opacity and NOx levels from incomplete combustion in affected cylinder during operation.
- Cold Start Issues: Difficult engine starting in low temperatures due to compromised fuel atomization on cylinder 5.
Probable Causes
- Wiring Harness Damage: Corroded or damaged injector harness causing short-to-ground or open circuit in cylinder 5 control wiring.
- Injector Solenoid Failure: Internal solenoid coil breakdown creating low resistance path reducing voltage feedback to ECM monitoring circuit.
- ECM Driver Circuit: Engine control module internal driver transistor failure preventing proper voltage supply to cylinder 5 injector.
- Connector Contamination: Water or debris in injector connector pins creating voltage drop and signal interference issues.
Advanced Technical Analysis
The ECM continuously monitors injector solenoid voltage through dedicated analog-to-digital converter channels, comparing actual voltage against expected parameters stored in calibration tables. When cylinder 5 injector voltage drops below 85% of nominal supply voltage for more than 200 milliseconds, the microcontroller triggers SPN 655 FMI 4. This monitoring occurs during both injection events and between-pulse periods to detect intermittent faults.
Voltage below normal conditions typically result from increased circuit resistance, creating voltage divider effects that reduce available solenoid operating voltage. The ECM applies debouncing algorithms with 50-millisecond sampling intervals to prevent false positives from electrical noise. Short-to-ground faults are distinguished from low voltage by monitoring current draw patterns, as shorts create immediate voltage collapse rather than gradual reduction.
Upon detecting this fault, the ECM implements cylinder-specific fuel delivery compensation by extending injection duration on remaining cylinders to maintain target torque output. Simultaneously, the system activates torque derate protocols, typically limiting engine output to 75% of rated power to prevent catastrophic engine damage from prolonged uneven combustion. The limp-home mode ensures vehicle operability while protecting engine integrity until repairs are completed.
Long-term diagnostic strategies involve trending voltage patterns across multiple key cycles to identify intermittent faults that may not trigger immediate codes. Technicians frequently encounter this fault after injector replacement when proper connector sealing procedures aren’t followed, allowing moisture ingress. Preventive maintenance includes regular harness inspection every 50,000 kilometers and connector dielectric grease application during routine service intervals to prevent corrosion-induced voltage drops.
Step-by-Step Troubleshooting Guide
- Voltage Measurement: Measure injector supply voltage at connector with DVOM, expecting 12V minimum during cranking operation.
- Resistance Testing: Test injector solenoid resistance between terminals, typical values range from 0.8 to 2.5 ohms specification.
- Harness Inspection: Visually inspect wiring harness for chafing, corrosion, or damage between ECM and cylinder 5 injector.
- ECM Verification: Perform ECM driver circuit test using oscilloscope to verify proper square wave output patterns.