SPN 5606 FMI 6: Meaning and Fix
SPN 5606 FMI 6 indicates the ECM detected a current above normal or a grounded circuit on the Adaptive Cruise Control Readiness Status input line. This fault often appears after a wiring harness repair near the front bumper or following a collision where the ACC radar sensor harness was pinched. The ECM reads a continuous 10b error condition, preventing the cruise control system from accepting ACC commands, even if the radar unit is functional. Technicians should inspect for chafed wires at chassis ground points.
Common Symptoms
- ACC Unavailable: Dashboard displays ‘ACC Not Ready’ or system refuses to engage despite radar sensor being operational.
- Cruise Disabled: Standard cruise control may also be disabled as a safety fallback when readiness status is invalid.
- Warning Lamp Active: Amber ACC warning lamp illuminated continuously; no diagnostic trouble codes for radar sensor itself.
- No Set Speed: Driver cannot set or resume any cruise speed; system remains passive regardless of button inputs.
Probable Causes
- Shorted Sensor Wire: ACC readiness signal wire shorted to chassis ground or battery positive, often near bumper connectors.
- Corroded Connector: Moisture ingress in the ACC radar module 8-pin connector causing pin-to-pin short circuit.
- ECM Input Fault: Internal ECM pull-up resistor failure or damaged input pin from previous overvoltage event.
- Aftermarket Accessory: Improperly installed lighting or winch wiring tied into ACC circuit causing current draw above normal.
Advanced Technical Analysis
The ECM continuously monitors the ACC readiness input line using a 5V pull-up resistor and a current-sensing circuit. A current exceeding the calibrated threshold (typically >2.5 mA) triggers FMI 6. The ECM then sets the status to ‘Error Condition’ (10b), blocking all ACC commands. This logic prevents false activation of adaptive cruise when the sensor circuit is compromised.
Electrical analysis reveals that a grounded circuit pulls the signal line below 0.8V, while a short to battery positive drives it above 4.5V. Both conditions cause the ECM’s analog-to-digital converter to report a current anomaly. The ECM also applies a 200 ms debouncing timer before logging the fault to avoid transient noise from relay switching.
Once FMI 6 is active, the ECM forces the ACC readiness status to ‘Error Condition’ (10b) and disables all adaptive cruise functions. Torque derate is not applied, but the vehicle reverts to standard cruise control only if the base cruise system is separately enabled. This safety fallback aligns with ISO 26262 ASIL B requirements for driver assistance systems.
Long-term prevention includes dielectric grease on ACC connectors and routing harnesses away from high-vibration areas. In a real workshop case, a MAN TGX repeatedly logged SPN 5606 FMI 6 after a DPF regeneration due to melted harness insulation near the exhaust. Replacing the harness section and adding heat shielding resolved the fault permanently.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check ACC radar harness from bumper to ECM for chafing, pinched wires, or melted insulation near exhaust.
- Measure Signal Voltage: With ignition on, measure voltage at ACC readiness pin; should be 4.5-5.0V open circuit, not 0V or battery voltage.
- Disconnect Module: Disconnect ACC radar module; if fault changes to open circuit, issue is in module or its connector.
- Check Ground Path: Verify ECM ground pins are clean and tight; high resistance ground can cause false current readings.