SPN 1807 FMI 19: Meaning and Fix
SPN 1807 FMI 19 indicates the Vehicle Dynamic Stability Control 2 message containing steering wheel angle data was received by the ECM with an error flag set. This fault commonly appears after a steering column repair or sensor replacement when the CAN bus wiring is disturbed, causing intermittent signal corruption. Technicians often see this code alongside other network faults, such as lost communication with the steering angle sensor.
Common Symptoms
- Stability Control Warning: Dashboard lamp for electronic stability program illuminates, disabling yaw control intervention.
- Erratic Steering Feel: Operator reports sudden changes in steering effort or a false center position during low-speed maneuvers.
- Reduced Engine Torque: ECM limits torque output by up to 40% as a safety measure to prevent loss of control.
- Failed Self-Test: Steering angle sensor fails calibration or returns implausible values during ignition-on sweep test.
Probable Causes
- Damaged Sensor Wiring: Chafed or corroded CAN H/L wires between steering angle sensor and chassis harness cause bit errors.
- Faulty Steering Sensor: Internal optical encoder failure or loose magnetic ring produces invalid angle data on the bus.
- ECM Software Mismatch: Incompatible calibration version after ECM replacement leads to misinterpretation of the J1939 message.
- Poor Ground Connection: High-resistance ground at sensor connector introduces noise, triggering the received network data in error flag.
Advanced Technical Analysis
The ECM monitors the J1939 message PG 0x00CF00200 (VDC2) for the steering wheel angle signal. When the CRC or bit-stuffing check fails, FMI 19 is set. The ECM uses a 100 ms debounce timer; if 3 consecutive messages are received with errors, the fault becomes active and the angle value is set to 0x7FFF (invalid). This logic prevents transient noise from triggering a false diagnostic.
Electrical analysis shows that the steering angle sensor operates on a 5V supply from the chassis harness. A voltage drop below 4.75V at the sensor connector due to corroded pins can cause the sensor to output a message with the data error flag set. The J1939 specification requires a differential voltage of at least 1.5V for a dominant bit; any imbalance below this threshold triggers the fault.
When FMI 19 is active, the ECM enters a safety fallback mode: the steering wheel angle is forced to 0 radians, disabling all yaw-rate-based stability functions. Engine torque is derated by 25% to 50% depending on OEM calibration (e.g., Mercedes-Benz OM471 engines limit torque to 1200 Nm). The vehicle remains drivable but with reduced dynamic stability.
Long-term prevention requires verifying the sensor mounting alignment per MAN TGS workshop manual procedure: the sensor must be centered within ±5° of mechanical zero. After sensor replacement, a steering angle calibration using a diagnostic tool (e.g., Bosch KTS 560) is mandatory. Real-world cases show that ignoring the calibration step causes the fault to reappear after a few key cycles.
Step-by-Step Troubleshooting Guide
- Inspect CAN Bus Wiring: Measure resistance between CAN H and CAN L at sensor connector; should be 60 ohms with ignition off.
- Check Supply Voltage: Verify 5.0 ±0.2V at sensor pin 1 relative to ground; low voltage indicates wiring fault.
- Perform Sensor Calibration: Use diagnostic tool to execute steering angle zero-point calibration with wheels straight ahead.
- Replace Steering Sensor: If wiring and voltage are correct, replace sensor and repeat calibration; clear fault and test drive.