SPN 652 FMI 31: Meaning and Fix
SPN 652 FMI 31 indicates a continuous condition exists on the cylinder 2 fuel injector circuit. The ECM detects a sustained fault, often an open or short, preventing proper solenoid actuation. This code commonly appears after a forced DPF regeneration when excessive heat damages the injector harness connector. Technicians frequently encounter this fault after replacing the ECM without verifying injector coil resistance against manufacturer specifications from Bosch or Deutz manuals.
Common Symptoms
- Rough Idle: Cylinder 2 misfire causes uneven engine speed at idle, noticeable vibration in cab.
- Power Loss: ECM derates torque to protect engine, reducing acceleration and load capacity.
- Excessive Smoke: Unburned fuel from cylinder 2 produces white or gray exhaust smoke.
- Increased Fuel Consumption: Misfire forces other cylinders to compensate, raising overall fuel usage.
Probable Causes
- Open Injector Circuit: Broken wire or corroded terminal at injector connector interrupts current flow.
- Shorted Injector Solenoid: Internal coil short to ground causes continuous current draw, ECM detects fault.
- ECM Driver Failure: Internal MOSFET on injector output channel stuck open or shorted per MAN service data.
- Harness Chafing: Worn insulation on engine harness contacts engine block, creating intermittent ground path.
Advanced Technical Analysis
The ECM microcontroller monitors the injector solenoid drive circuit using a current-sense resistor and feedback comparator. FMI 31 triggers when the measured current remains outside the expected window for over 2 seconds, indicating a hard fault. Bosch ED17 systems use a debouncing timer to prevent false triggers from electrical noise. The condition must persist across multiple engine cycles to set the code active.
Electrical analysis reveals that an open circuit results in zero current flow, while a short to ground causes excessive current beyond the ECM’s programmable limit. Deutz factory manuals specify a coil resistance of 0.5–1.2 ohms at 20°C. Any deviation beyond ±15% triggers the condition. The ECM’s low-side driver tests the circuit with a pre-drive pulse before each injection event.
Upon detecting the condition, the ECM activates a safety fallback: it disables the injector for cylinder 2 and enters a torque derate of 25–40%. The engine management system switches to a limp-home mode, limiting RPM to 1500. Mercedes-Benz OM471 engines further log an event in the non-volatile memory for post-fault analysis by workshop software like Xentry.
Long-term diagnostic strategy involves performing a resistance check at the ECM connector pinout per SAE J1939-73. Real-world workshops often find that a new injector harness resolves the fault, especially after engine overhaul. Technicians should also inspect the injector return spring for damage, as a stuck-open needle can mimic an electrical fault. Always clear the code after repair and verify with a cylinder cutout test.
Step-by-Step Troubleshooting Guide
- Inspect Connector: Check cylinder 2 injector connector for corrosion, bent pins, or loose terminal retention.
- Measure Resistance: Use multimeter across injector pins; expect 0.5–1.2 ohms. Compare to opposite cylinder.
- Test ECM Output: Back-probe ECM connector pin for cylinder 2; check for battery voltage during key-on.
- Perform Wiggle Test: Flex harness near engine while monitoring scan tool; fault must not flicker on/off.