What does it mean?
B1291 is a Body (airbags, seat belts, body electronics) fault code. It indicates: Short to B+ in Light System Communication Bus Malfunction .
Symptoms
- Headlamps, tail lamps or adaptive lighting stop responding or default to a fixed state
- One or more lamp modules cannot be reached with a scan tool
- Lighting warning message; automatic high beam and levelling are disabled
- Lamps may stay on after the ignition is switched off, or fail to come on at all
- Battery drain if lamps remain energised overnight
Likely causes
- Bus wire chafed against a battery-positive feed or an exposed terminal in the lighting harness
- Water intrusion into a headlamp or tail lamp connector bridging the bus pin to a supply pin
- Damaged loom in a front section repaired after a collision or after bulb replacement
- Failed lamp control module or ballast pulling the line to supply
- Aftermarket lamp or LED conversion spliced into the wrong wire
- Corroded connector in the cowl or wheel arch, both permanently damp areas
How to fix it
- Scan every module and record which lighting nodes are missing, that narrows the fault to one branch
- Measure the bus line voltage with the ignition on; a line sitting at battery voltage confirms the short
- Disconnect lamp modules one at a time and watch for the moment bus voltage returns to normal
- Inspect headlamp and tail lamp connectors for moisture, melted pins and corrosion
- Review any recent aftermarket lighting work and remove improper splices
- Repair the chafed harness section properly with sealed, insulated joints
- Replace the failed lamp module or ballast if it is the node loading the line, then clear codes and retest all lighting functions