SPN 3661 FMI 31: Meaning and Fix
SPN 3661 FMI 31 indicates that the ECM has detected a continuous, active electrical fault on the second actuator of the fuel injector for cylinder 3. This condition exists without intermittent behavior, often triggered by a short-to-battery or open circuit in the injector harness. Technicians frequently encounter this code after replacing the ECM or performing a forced DPF regeneration, when residual debris or pin corrosion disrupts the actuator circuit.
Common Symptoms
- Rough idle: Cylinder 3 misfire leads to uneven idle, noticeable vibration, and audible knocking at low RPM.
- Power loss: Engine torque derate of up to 40% under load due to ECM safety response to the fault.
- Check engine light: Red or amber MIL illuminates immediately; fault remains active until circuit is restored.
- Increased smoke: Unburned fuel from cylinder 3 produces white or gray exhaust, especially during cold starts.
Probable Causes
- Short to battery: Worn insulation on actuator 2 wire contacts battery voltage, causing continuous fault detection.
- Open circuit: Broken wire or corroded pin at connector C of the injector harness prevents actuator current flow.
- ECM driver failure: Internal short in the ECM output stage for cylinder 3 actuator 2, common after voltage spikes.
- Injector internal short: Coil winding breakdown in the injector actuator creates a low-impedance path, triggering the fault.
Advanced Technical Analysis
The ECM microcontroller continuously monitors the actuator circuit for voltage and current compliance. When the measured feedback deviates from the expected range for more than 500 ms, the diagnostic state machine sets SPN 3661 FMI 31 as a latched condition. This logic prevents false triggers from transient noise but ensures persistent faults are captured.
Electrical analysis of the actuator 2 circuit reveals a typical resistance of 0.5–1.5 Ω at 20°C. A short-to-battery raises the measured voltage above 5V on the sense line, while an open circuit causes the pull-up resistor to clamp at 5V. The ECM debouncing timer confirms the fault across three consecutive ignition cycles before activating the lamp.
Upon detecting this condition, the ECM enters a safety fallback mode: fuel injection to cylinder 3 is fully disabled, and the engine torque is derated by 30–50%. The engine management system also reduces boost pressure and adjusts injection timing on remaining cylinders to minimize vibration and exhaust temperature.
Long-term diagnostic strategy includes performing a resistance measurement at the injector connector and comparing it to factory specifications (e.g., Bosch 0.9 Ω ±0.1 Ω). Real-world workshop examples show this fault often recurs after engine rebuilds where harness routing is disturbed. Preventive replacement of injector sub-harnesses every 5000 operating hours is recommended by MAN factory manuals.
Step-by-Step Troubleshooting Guide
- Visual harness check: Inspect wiring from ECM to cylinder 3 for chafing, burns, or corrosion, especially near exhaust manifold.
- Resistance measurement: Measure actuator 2 pins at injector; expected 0.8–1.2 Ω. Out-of-range indicates short or open.
- ECM connector test: Back-probe ECM pin for cylinder 3 actuator 2; check for 12V supply or ground short with multimeter.
- Injector substitution: Swap injector with a known-good cylinder; if fault moves, replace injector; if not, check ECM driver.