What does it mean?
B1909 is a Body (airbags, seat belts, body electronics) fault code. It indicates: Rear sensor signal fault .
Symptoms
- Air bag warning lamp on and restraint functions partly disabled
- Curtain bags, side bags or rear pretensioners inhibited depending on the system layout
- Fault appears right after interior trim removal or rear speaker installation
- Code may return within a short drive after being cleared
- No effect on engine performance, only a persistent safety warning
Likely causes
- Open circuit in the sensor signal or supply wire, often where the loom flexes near the rear seat or wheel arch
- Connector unplugged or not latched after boot trim, rear seat or audio work
- Corroded terminals after water reached the rear of the vehicle
- Sensor internally failed and no longer transmitting
- Sensor loose on its mounting, so its output does not match vehicle motion
- Poor ground or low supply voltage at the rear sensor
- Restraint control module input fault
How to fix it
- Observe air bag safety precautions before disturbing any restraint wiring
- Read the codes and freeze frame and establish whether the fault is permanent or intermittent
- Confirm the sensor connector is fully seated and locked, as it is often left unplugged after trim work
- Measure supply voltage, ground and signal continuity at the sensor connector and back to the module
- Wiggle the loom near the rear seat, wheel arch and trim clips while monitoring the circuit to catch an intermittent open
- Repair the harness with correct terminals, then dry and protect the connector
- Replace the restraint control module only after all of the above have been checked