SPN 558 FMI 5: Meaning and Fix
This fault indicates the ECM detects an open circuit or current below normal on the Accelerator Pedal 1 Low Idle Switch signal line. The switch, defined per SAE J1843, should read 01b when the pedal is at idle. A broken wire or corroded connector at the pedal assembly often triggers this code. Technicians frequently encounter this after engine harness chafing near the firewall or following a pedal replacement without proper calibration.
Common Symptoms
- No idle validation: ECM cannot confirm pedal is at idle; engine may stall or surge when releasing throttle.
- Reduced engine power: Torque derate active; vehicle may not exceed 1200 rpm as a safety fallback.
- Check engine lamp on: Red or amber MIL illuminated on dash, often with active diagnostic trouble code.
- Hard pedal feel: Digital accelerator signal ignored; ECM uses default map causing unresponsive throttle tip-in.
Probable Causes
- Open signal wire: Break in circuit between pedal sensor pin B and ECM connector due to harness flexing.
- Corroded connector: Moisture ingress at pedal or ECM side causing high resistance or open circuit.
- Failed pedal switch: Internal mechanical failure of the low idle switch contact inside the pedal assembly.
- ECM pin damage: Fretting corrosion or bent terminal at ECM connector J2 pin 12 causing intermittent open.
Advanced Technical Analysis
The ECM microcontroller monitors the low idle switch state via a digital input with a pull-up resistor to 5V. When the switch closes (pedal at idle), the signal is pulled low to ground. An open circuit leaves the input floating at 5V, which the ECM interprets as 00b (not in idle) or 10b (error) if the debounce timer expires. This logic aligns with SAE J1939-71 and Bosch EDC7 UC31 specifications.
Electrical analysis shows the switch circuit must have less than 100 ohms resistance to ground when closed. An open circuit exceeds this threshold, causing the ECM to set FMI 5. The debounce timer, typically 100 ms per MAN diesel factory standards, prevents transient noise from triggering false faults. Persistent open for >2 seconds latches the code into active memory.
Upon detecting the open circuit, the ECM activates a torque derate strategy, limiting engine power to 30% of demand. Per Mercedes-Benz OM471 service literature, the vehicle enters limp-home mode with maximum speed capped at 20 km/h. The ECM also disables cruise control and idle speed governors to prevent unintended acceleration scenarios.
Long-term prevention requires inspecting the pedal harness for chafing at the steering column grommet, a common failure point on Deutz TCD 2013 engines. Technicians should measure pin-to-pin resistance between pedal connector and ECM using a milliohm meter. Replacing the pedal assembly without checking the harness often leads to recurring faults, as documented in Bosch workshop bulletins.
Step-by-Step Troubleshooting Guide
- Visual inspection: Check pedal connector and ECM harness for damage, corrosion, or bent pins. Repair as needed.
- Resistance measurement: Measure between pedal signal pin and ECM pin; should be < 0.5 ohms. Open indicates wire break.
- Pedal switch test: With pedal released, measure continuity to ground; should be < 10 ohms. Replace if open.
- ECM pin check: Back-probe ECM connector; verify 5V pull-up present. If missing, repair ECM supply circuit.