What does it mean?
B1878 is a Body (airbags, seat belts, body electronics) fault code. It indicates: Driver-Side Pre-Tensioner Seat Belt Circuit Short To Power Supply .
Symptoms
- Air bag warning lamp stays on from the moment the ignition is switched on
- Driver belt pretensioner disabled, and often other restraint outputs are inhibited as well
- Fault may appear after seat removal, carpet cleaning or an accessory installation
- Seat heater or seat motor on the same side may also misbehave or blow its fuse
- Restraint warning message on the driver display
Likely causes
- Insulation rubbed through under the seat so a pretensioner wire touches a powered circuit such as a seat heater or seat motor feed
- Harness crushed by the seat rail or a seat mounting bolt during a seat installation
- Water and salt in the under seat connector bridging a firing pin to an adjacent powered pin
- Incorrect wiring from an aftermarket seat, heater kit or audio installation routed alongside the restraint harness
- Damaged connector housing that lets a terminal touch a neighbouring circuit
- Restraint control module internal short on the driver pretensioner output
How to fix it
- Follow air bag safety procedure before disconnecting anything: ignition off, battery off, stand time completed
- Read the codes with freeze frame data and note whether the fault appeared after recent seat or interior work
- Disconnect the pretensioner at the seat and check the harness side for voltage with the ignition on; any voltage confirms a short to power upstream
- Trace the harness along the seat rails and under the carpet looking for crush marks, melted insulation and contact with a powered wire
- Open the under seat connector and look for moisture, salt residue and terminals bridged by corrosion
- Remove any non factory wiring that has been routed with or spliced into the restraint harness
- If the harness is proven clean and the short is still present with everything disconnected, replace the restraint control module