SPN 651 FMI 31: Meaning and Fix
SPN 651 FMI 31 indicates that the Engine Control Module (ECM) has detected a persistent electrical or mechanical fault on the Cylinder 1 fuel injector circuit, with the condition actively present. This code commonly appears after a forced DPF regeneration or after replacing the ECM, when high current draw or a shorted injector solenoid causes the ECM to lock out the injector. Technicians frequently encounter this fault on Deutz and MAN engines after wiring harness chafing near the valve cover.
Common Symptoms
- Rough Idle: Engine runs unevenly at idle due to no fuel delivery from Cylinder 1.
- Misfire Under Load: Noticeable power loss and vibration when engine is under heavy load.
- Increased Exhaust Smoke: White or black smoke from unburned fuel in Cylinder 1.
- Check Engine Lamp On: Red or amber MIL continuously illuminated on the dashboard.
Probable Causes
- Shorted Injector Solenoid: Internal short circuit in Cylinder 1 injector coil causing high current draw.
- Harness Chafing: Worn insulation on injector wiring near cylinder head grounding points.
- ECM Driver Failure: Failed output driver inside ECM for Cylinder 1 injector circuit.
- Corroded Connector: Moisture ingress at injector or ECM connector pins causing intermittent contact.
Advanced Technical Analysis
The ECM monitors the injector solenoid current profile during each injection event. A condition exists fault (FMI 31) is set when the measured current exceeds or falls below the calibrated threshold for more than 10 consecutive cycles. This triggers a permanent diagnostic trouble code (DTC) until manually cleared. On Bosch EDC7 systems, the ECM also logs the peak current value for post-mortem analysis.
Electrical analysis reveals that a short-to-ground or short-to-battery on the injector line causes the ECM’s high-side driver to enter current limit mode, often at 4.5 A. The debouncing timer in the ECM (typically 5 seconds) must elapse before the fault is confirmed. If the condition clears, the code may become inactive but remains stored. MAN engines use a 2.5 A threshold for injector circuit faults.
When the ECM detects the condition, it deactivates Cylinder 1 injection completely to prevent driver damage. This triggers a torque derate of up to 40% and a speed limit of 1500 RPM. The ECM also disables cylinder balancing for that bank, causing increased vibration. On Mercedes-Benz OM471 engines, the ECM may attempt one recovery cycle per key cycle before locking out the injector.
Long-term prevention includes dielectric greasing of injector connectors and routing harnesses away from hot surfaces. A real-world example: on a Deutz TCD 2015 engine, a technician found the injector harness chafed against the valve cover gasket rail, causing intermittent shorts. Replacing the harness and injector sub-harness resolved the fault permanently. Always verify injector resistance (0.5–1.2 Ω) before ECM replacement.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check Cylinder 1 injector harness for chafing, burns, or corrosion near cylinder head.
- Resistance Measurement: Measure injector solenoid resistance at ECM pins; should be 0.5–1.2 Ω at 20°C.
- ECM Power Cycle: Cycle ignition off for 30 seconds; recheck if fault clears before component replacement.
- Injector Swap Test: Swap injector with Cylinder 2; if fault moves, replace the injector.