SPN 929 FMI 5: Meaning and Fix
This fault indicates the tire location identification circuit (high/low order nibble) has current below normal or an open circuit. The ECM detects a missing or high-impedance signal from the tire pressure monitoring system sensor. Commonly appears after a tire rotation or replacement when the sensor wiring harness is accidentally pinched or left unplugged, or after a forced DPF regeneration where heat damages the sensor connector.
Common Symptoms
- Missing Tire ID: ECM reports tire position as FFh (not available) for one or more wheels on the J1939 bus.
- Warning Lamp: Amber warning lamp illuminates on dash; no active tire pressure data for the affected location.
- No TPMS Data: Tire Condition Message 1 PG transmits zero or erratic pressure/temperature values for that wheel.
- Intermittent Fault: Fault appears only during vibration or cold starts; clears after key cycle when connector re-seats.
Probable Causes
- Open Sensor Wire: Broken conductor inside TPMS sensor pigtail or chassis harness near wheel well due to chafing.
- Corroded Connector: Pin corrosion at sensor or ECM connector causing high resistance and current below normal threshold.
- Faulty Sensor: Internal TPMS sensor failure; open circuit in the ASIC or antenna coil, common after impact damage.
- ECM Input Fault: Damaged ECM input pin or internal pull-up resistor failure causing open circuit detection.
Advanced Technical Analysis
The ECM monitors the tire location sensor line via a pull-up resistor to 5V. When current falls below the calibrated threshold (typically <2 mA), the FMI 5 is set. The microcontroller samples the analog voltage; an open circuit yields Vcc, triggering the fault. Debouncing timer of 500 ms prevents false triggers from transient noise.
Electrical analysis shows that a broken wire or corroded pin creates an open loop. The ECM’s input impedance is ~10 kΩ; with no sensor load, voltage rises above 4.5V. The J1939 message then sets the tire position nibble to FFh. Real-world data from Deutz workshop manuals confirm this pattern after water ingress in wheel-end connectors.
When FMI 5 is active, the ECM defaults to a ‘tire not available’ state. No torque derate occurs, but the TPMS system disables pressure warnings for that wheel. The fault remains latched until the circuit current returns above 3 mA for three consecutive key cycles, per Bosch ED17 controller logic.
Long-term prevention includes dielectric grease on connectors and securing harnesses away from heat sources. Technicians often find this fault after tire replacements where the sensor bracket is overtightened, crushing the wire. Using a breakout box to measure resistance between sensor and ECM pins (should be <5 Ω) isolates the open circuit quickly.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check TPMS sensor connector and harness from wheel well to chassis for cuts, corrosion, or loose pins.
- Resistance Test: Disconnect sensor; measure resistance between signal and ground pins. Open circuit >1 MΩ confirms broken wire.
- Backprobe ECM: With ignition on, measure voltage at ECM input pin. Steady 5V indicates open; 0V indicates short to ground.
- Sensor Bypass: Temporarily jumper a known-good TPMS sensor. If fault clears, replace original sensor and connector.