SPN 558 FMI 31: Meaning and Fix
SPN 558 FMI 31 signals that the accelerator pedal 1 low idle switch reports a continuous error condition (binary 10) to the ECM via the EEC2 PG. This fault commonly appears after a forced DPF regeneration when the pedal is not fully released, or after an ECM replacement where the switch calibration is lost. The ECM cannot confirm if the pedal is at idle, which may limit engine response and throttle authority.
Common Symptoms
- Idle validation failure: ECM cannot confirm low idle state; engine may not return to idle when pedal released.
- Reduced throttle response: Accelerator pedal input is partially ignored; engine power limited to limp-home mode.
- Fault lamp illuminated: Red or amber warning lamp activates immediately due to persistent switch error condition.
- Erratic idle speed: Engine hunts or surges at idle because ECM receives conflicting switch and pedal position signals.
Probable Causes
- Faulty low idle switch: Internal mechanical failure or contamination prevents switch from toggling correctly between states.
- Wiring short or open: Signal line shorted to ground or battery, or open circuit, causing continuous error bit.
- ECM software mismatch: After ECM replacement, incorrect calibration fails to recognize switch state transitions.
- Pedal assembly damage: Physical damage to pedal mechanism or connector corrosion locks switch in error state.
Advanced Technical Analysis
The ECM continuously monitors the low idle switch signal as part of the EEC2 message. Per SAE J1939-71, the switch must transition from 01b (idle) to 00b (not idle) within a defined debounce window. If the ECM reads 10b (error) for more than 500 ms, it sets FMI 31. This logic prevents transient noise from triggering false faults but requires stable switch contact closure.
Electrical analysis shows the switch is typically a normally open contact pulled high via a 10 kΩ resistor to 5 V. When the pedal is released, the switch closes to ground, pulling the signal low. A short to battery or an open ground path will produce a continuous high or floating state, interpreted as error 10b. Oscilloscope testing at the ECM pin can reveal noise or stuck voltage levels.
Upon detecting SPN 558 FMI 31, the ECM enters a safety fallback: it disables the low idle switch validation and relies solely on the accelerator pedal position sensor for idle detection. Torque output is derated to 30% of maximum, and cruise control is inhibited. This strategy is based on Mercedes-Benz factory guidelines to prevent unintended acceleration if the switch fails.
Long-term prevention requires verifying switch calibration after any ECM software update or pedal assembly replacement. Real-world workshops report that this fault often recurs on vehicles with aftermarket pedal spacers that alter switch actuation travel. Technicians should measure switch stroke with a digital caliper and adjust pedal stop bolts per OEM specifications to ensure proper contact closure.
Step-by-Step Troubleshooting Guide
- Read freeze frame data: Record engine speed and pedal position at fault onset to confirm switch error persists.
- Test switch continuity: Disconnect pedal connector; measure resistance between switch pins while pressing/releasing pedal.
- Check for voltage drop: With ignition on, measure signal voltage at ECM pin; should toggle 0 V idle to 5 V not-idle.
- Inspect pedal mechanism: Verify pedal returns fully to stop; clean debris and confirm switch plunger depresses completely.