SPN 929 FMI 14: Meaning and Fix
SPN 929 FMI 14 signals that the Tire Location parameter in the Tire Condition Message 1 PG is reporting a value of FFh (not available) or an invalid position number per SAE J1939/71. This fault often appears after a tire rotation or when a new ECU is installed without reconfiguring the tire position mapping. Technicians see this when a TPMS module fails to assign left-to-right or front-to-back coordinates, leaving the system unable to correlate pressure data to specific wheel ends.
Common Symptoms
- No Tire ID: Dashboard displays dashes or ‘N/A’ for individual tire positions instead of pressure or temperature values.
- False Alarms: Intermittent low-pressure warnings appear on random positions without actual pressure loss.
- Data Gap: J1939 datalogger shows FFh for tire location bytes while other sensor data is valid.
- Reset Loop: TPMS controller repeatedly cycles power trying to re-learn tire positions after ignition on.
Probable Causes
- Unconfigured Module: New or replacement TPMS ECU not programmed with vehicle-specific tire position map.
- Sensor Swap: Tire rotation performed without updating position IDs in the TPMS or gateway module.
- Corrupted Calibration: J1939 configuration parameter for tire location table lost due to low battery or flash error.
- CAN Bus Noise: Electrical interference corrupts the tire position nibble, causing FFh fallback on the bus.
Advanced Technical Analysis
The ECM monitors the Tire Location byte (PGN 0xFD06) as defined in SAE J1939/71. The low nibble encodes lateral position (1=left, 2=right) and the high nibble longitudinal position (1=front, 2=rear). FMI 14 triggers when the received value is FFh or outside 11h–22h range, indicating the source (typically a TPMS or gateway) has not been properly initialized or has lost its mapping table. The controller sets the fault without entering a derate, but inhibits tire pressure data transmission until a valid location is learned.
Electrically, this fault is rarely a wiring issue because the location data is transmitted digitally over CAN. However, if the gateway module experiences a brownout or EEPROM corruption, the stored tire position table may revert to factory defaults. The J1939 protocol requires the transmitting device to output FFh when location is unknown. Debouncing timers are not applicable here since FMI 14 is a special instructions code—it is set immediately upon reception of an invalid nibble and cleared only after a valid position is received for 10 consecutive messages.
Safety fallback is minimal: the engine and drivetrain continue normal operation. The TPMS system may default to a global ‘all tires’ alarm rather than position-specific warnings. In vocational trucks with dual rear axles, this can lead to undetected slow leaks on inner duals because the system cannot isolate the faulty wheel end. No torque derate or speed limit is imposed, but fleet management systems may flag the vehicle for maintenance due to incomplete tire data.
Long-term prevention requires a standardized re-learn procedure after every tire rotation or ECU swap. For example, on a Mercedes-Benz Actros with TPMS, technicians must use XENTRY to assign each sensor ID to its wheel position. On Deutz-powered off-highway equipment, the TECU must be reconfigured via the diagnostic tool. Real-world example: a fleet of MAN TGX trucks repeatedly triggered this code after tire rotations because the workshop skipped the TPMS learn step, causing false alarms until the mapping was corrected.
Step-by-Step Troubleshooting Guide
- Verify Position Map: Use diagnostic tool to read tire location configuration; ensure each sensor ID is assigned to correct wheel end.
- Cycle Ignition: Power cycle the vehicle and wait 60 seconds; if fault clears, a temporary bus glitch caused FFh.
- Inspect Gateway: Check for low voltage or corrosion at TPMS gateway module connector that could corrupt stored mappings.
- Re-Learn Sensors: Initiate automatic or manual tire position re-learn procedure per OEM service manual (e.g., using TPMS trigger tool).