SPN 558 FMI 13: Meaning and Fix
SPN 558 FMI 13 indicates the Accelerator Pedal 1 Low Idle Switch is out of calibration. This switch, defined per SAE J1843, signals idle or not-idle states. When out of calibration, the ECM detects an invalid voltage range or switch transition error. Technicians often see this after a pedal replacement without proper relearn, or after a battery disconnect that resets learned positions. The ECM may log this fault during key-on self-test if the switch signal does not match the pedal position sensor.
Common Symptoms
- Erratic Idle: Engine may not return to low idle or hunts at idle due to incorrect switch state.
- Reduced Power: ECM limits torque output to protect drivetrain when pedal switch calibration is invalid.
- Stall on Decel: Vehicle stalls when throttle is released because ECM misinterprets idle switch signal.
- No Start Condition: Some ECMs inhibit starting if low idle switch calibration fails during key-on self-test.
Probable Causes
- Pedal Replacement: New pedal assembly not calibrated via ECM relearn procedure per manufacturer guidelines.
- Battery Disconnect: Loss of power erases learned pedal position and switch calibration values in ECM.
- Sensor Drift: Aging potentiometer or Hall-effect sensor inside pedal drifts beyond calibration tolerance.
- ECM Software Glitch: Firmware bug or corruption loses stored calibration data, requiring reflash or reset.
Advanced Technical Analysis
The ECM monitors the low idle switch via a dedicated digital input line with a pull-up resistor. Under normal operation, the switch state must match the pedal position voltage within a defined window. When out of calibration, the ECM compares the learned idle switch point against the actual signal and detects a discrepancy, setting FMI 13. This logic is defined in SAE J1939-71 and implemented in Bosch MD1 and EDC17 ECUs.
Electrically, the switch circuit uses a debouncing timer of 10–50 ms to prevent noise from triggering false faults. An out-of-calibration condition typically arises when the switch voltage at idle exceeds 4.5 V or drops below 0.5 V for more than 100 ms. MAN and Deutz factory manuals specify a calibration procedure that involves cycling the pedal from idle to full throttle three times within 30 seconds after ECM reset.
The ECM activates a torque derate of approximately 25% when this fault is active, as per Mercedes-Benz OM471 and OM936 service literature. Additionally, the engine management system may force a fixed idle speed of 800–900 RPM to allow safe vehicle operation. The fault is stored as active until the calibration is successfully completed and the ECM verifies the switch transition within the learned window.
Long-term prevention includes performing a pedal calibration after every ECM software update or battery replacement. In real-world workshops, this fault is frequently encountered after installing a remanufactured pedal assembly without running the relearn routine. Technicians should also inspect the pedal connector for corrosion or bent pins, as intermittent contact can corrupt the calibration data stored in the ECM’s non-volatile memory.
Step-by-Step Troubleshooting Guide
- Perform Relearn: Cycle ignition on, depress pedal fully, release, then idle for 10 seconds per OEM procedure.
- Check Supply Voltage: Measure 5V reference and ground at pedal connector; verify within 4.75–5.25V.
- Inspect Wiring: Look for chafed or loose wires at pedal and ECM connector; repair as needed.
- Replace Pedal Assembly: If calibration fails and wiring is intact, replace pedal and repeat relearn procedure.