SPN 929 FMI 2: Meaning and Fix
SPN 929 FMI 2 indicates the Tire Location parameter in the Tire Condition Message 1 PG is erratic, intermittent, or incorrect. This fault often appears after a wheel-end service or tire rotation when the position identification wiring is disturbed. Technicians commonly see this code on trucks with TPMS systems after a sensor replacement if the new sensor ID is not properly linked to the correct wheel position. The ECM detects invalid or jumping position numbers, triggering the diagnostic.
Common Symptoms
- Erratic Tire ID: Tire pressure or temperature readings jump between wheel positions on the display.
- Missing Wheel Data: One or more tire sensors show no data while others report correctly on the J1939 bus.
- False Warnings: Dashboard alerts for tire pressure faults that do not match actual measured tire conditions.
- Communication Gaps: Intermittent loss of tire condition messages during vehicle vibration or turns.
Probable Causes
- Wiring Harness Damage: Chafed or corroded wires in the wheel-end sensor harness cause intermittent position data.
- Sensor ID Mismatch: Replaced tire sensor not programmed to the correct wheel position in the ECU.
- Connector Corrosion: Moisture ingress at the axle-mounted connector creates high-resistance intermittent faults.
- ECM Software Glitch: Firmware bug in the tire location mapping logic causes erratic position number assignment.
Advanced Technical Analysis
The ECM monitors the Tire Location byte (0–15) in the Tire Condition Message 1. It expects a stable position number consistent with the vehicle configuration. When the microcontroller detects a value changing more than once per 100 ms without a valid rotation event, it sets FMI 2. This debouncing timer prevents false flags from momentary noise but catches real erratic behavior.
Electrically, the fault often stems from a voltage drop across the sensor supply line below 9 V during vibration. The sensor then resets and transmits a default position of FFh (not available). The ECM interprets this intermittent FFh as erratic data. Checking supply voltage at the sensor connector under load is critical for isolating the root cause.
The ECM does not derate engine power for this fault, but it disables tire pressure-based load management and may log a companion code for the affected sensor. In some OEM implementations, the vehicle enters a limited speed mode of 55 km/h if the fault persists over 10 minutes, as per MAN and Mercedes-Benz safety fallback strategies.
Long-term prevention requires proper sensor ID registration after any tire service. Using a diagnostic tool to perform a wheel position learn sequence ensures the J1939 database is updated. In workshops, this code frequently appears after a sensor swap using a non-OEM tool that omits the position mapping step, leading to repeat customer complaints.
Step-by-Step Troubleshooting Guide
- Scan for Codes: Read all active and inactive SPNs with a J1939 tool to identify the erratic tire sensor.
- Inspect Connectors: Check the axle-mounted connector for corrosion, bent pins, or moisture contamination.
- Verify Sensor ID: Use the TPMS tool to confirm each sensor ID matches its assigned wheel position.
- Perform Learn Cycle: Initiate a wheel position learn sequence via OEM software to remap sensor locations.