SPN 1807 FMI 6: Meaning and Fix
SPN 1807 FMI 6 indicates the steering wheel angle sensor circuit has current above normal or a grounded condition. This fault commonly appears after a collision repair where the steering column harness was pinched or after a technician accidentally shorts the sensor wires during ECM replacement. The ECM detects excessive current draw and sets the code, disabling vehicle dynamic stability control functions until resolved.
Common Symptoms
- Stability Control Off: VDSC2 system deactivates; warning lamp illuminates on dash indicating loss of yaw control.
- Erratic Steering Angle: Dash display shows random steering angle values or stuck at zero regardless of wheel position.
- Torque Derate Active: ECM reduces engine power to 50% as a safety fallback to prevent unstable handling.
- No Communication: Diagnostic tool reports no data from steering angle sensor on J1939 bus during active fault.
Probable Causes
- Sensor Supply Short: 5V reference wire shorted to ground within the sensor harness or connector.
- Internal Sensor Failure: Short circuit inside the steering angle sensor module due to moisture ingress or mechanical damage.
- Pinched Harness: Wiring chafed against steering column bracket causing direct ground contact.
- ECM Driver Fault: Internal short in the ECM’s sensor supply driver, often after voltage spike events.
Advanced Technical Analysis
The ECM monitors the steering angle sensor’s current draw via a precision shunt resistor. Under normal operation, the sensor draws 10-20 mA. When FMI 6 sets, the ECM has detected current exceeding 50 mA for more than 200 ms, indicating a hard short to ground. The ECM then latches the fault and disables the sensor’s 5V supply to protect the internal driver.
A debouncing timer prevents transient spikes from triggering false faults. The ECM requires the overcurrent condition to persist across three consecutive 100 ms sampling windows. This means a brief short during key-on self-test will not set the code, but a continuous short from a pinched wire will. Real-world examples include harness damage from steering column adjustments after dashboard removal.
Once the fault is active, the ECM sets a default steering angle of 0° and disables all VDSC2 interventions. Engine torque is derated to 50% and vehicle speed may be limited to 30 km/h. The system will not re-enable until the fault is cleared and a successful sensor calibration is performed with the steering wheel centered.
Long-term prevention includes routing the sensor harness away from moving steering components and using split-loom conduit. Technicians frequently encounter this fault after replacing the ECM without transferring the sensor wiring correctly—always verify pin continuity before powering the new module. A dielectric grease application on connector seals reduces moisture-related shorts.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check steering column harness for pinch points, chafing, or exposed copper near metal brackets.
- Measure Resistance: Disconnect sensor; measure between 5V and ground pins. Expect >10 kOhm; if <10 Ohm, locate short.
- Isolate Sensor: Unplug sensor and jumper 5V supply to signal line. If current drops, replace sensor module.
- ECM Output Test: With sensor disconnected, measure ECM pin voltage. 5V ±0.2 indicates ECM ok; 0V suggests driver failure.