SPN 107 FMI 31: Meaning and Fix
This fault indicates the engine air filter 1 differential pressure sensor has detected a condition exists, often a sustained high pressure drop. Technicians commonly encounter this after a forced DPF regeneration or operation in heavy dust environments. The ECM logs the fault when measured pressure exceeds a calibrated threshold for a defined period, typically above 7.5 kPa at rated airflow per SAE J1939.
Common Symptoms
- Reduced Power: Engine enters torque derate, limiting power output to protect the engine from excessive restriction.
- High Exhaust Temp: Increased intake restriction causes higher exhaust temperatures, potentially triggering related thermal faults.
- Poor Fuel Economy: Engine labors to draw air, increasing fuel consumption by up to 5% under load per Deutz workshop data.
- Audible Whistle: Air whistling through a restricted filter or leaking sensor line is often reported by operators.
Probable Causes
- Clogged Filter: Primary cause: accumulated debris exceeding 6 kPa differential pressure at rated flow per MAN guidelines.
- Sensor Failure: Diaphragm rupture or internal short in the differential pressure sensor, common after water ingress.
- Wiring Fault: Open circuit, short to ground, or chafed wires at the sensor harness, often near the air cleaner housing.
- Blocked Drain Valve: Stuck open or closed dust ejector valve causes false high readings due to moisture accumulation.
Advanced Technical Analysis
The ECM monitors the differential pressure sensor signal (0.5-4.5 V linearized to 0-10 kPa) at 100 Hz. A condition exists (FMI 31) is set when the signal remains above 7.5 kPa for more than 10 seconds of accumulated engine run time, per Bosch ED17 calibration tables. The ECM then activates a diagnostic trouble code and may illuminate the MIL.
Electrical analysis reveals the sensor uses a 5 V reference from the ECM. A short to ground pulls the signal below 0.5 V, while an open circuit floats above 4.5 V. The FMI 31 specifically indicates the signal is valid but indicates a real condition—not a circuit fault. Debouncing timers prevent false triggers from transient pressure spikes during sudden throttle changes.
Upon activation, the ECM triggers a gradual torque derate of 25% over 30 seconds to allow safe vehicle operation. If the condition persists for 60 minutes, a further derate to 50% occurs. This strategy, documented in Mercedes-Benz OM471 service manuals, prevents engine damage from excessive restriction while enabling the vehicle to reach a service bay.
Long-term prevention includes verifying the air filter restriction indicator matches sensor readings. In one workshop case, a truck repeatedly set SPN 107 FMI 31 after a new ECM installation due to mismatched sensor calibration—reprogramming the ECM with the correct sensor curve resolved it. Regular cleaning of the pressure tap ports and replacing the filter at 5 kPa differential pressure avoids recurrence.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check air filter for visible clogging and ensure the housing is sealed and drain valve operates freely.
- Sensor Test: Disconnect sensor, blow 8 kPa through the high side port; verify signal voltage rises linearly per spec.
- Harness Check: Measure resistance between sensor connector and ECM pins; look for breaks or shorts with a multimeter.
- Live Data Scan: Monitor differential pressure with a J1939 tool at idle and full throttle; compare to restriction gauge reading.