What does it mean?
B2157 is a Body (airbags, seat belts, body electronics) fault code. It indicates: Driver's MPCS Unit Lost Communication with Gauge Control Module (ILLUMI Message) .
Symptoms
- Interior movement is no longer detected when the alarm is armed
- False alarms, often at night or when the temperature changes
- Alarm arms only in a reduced mode, or the indicator flashes an unusual pattern
- Warning message about the security system
- Front sensor status frozen in live data
- Code sets again immediately after clearing
Likely causes
- Sensor connector not fully seated after windscreen, mirror or headliner work
- Wiring pinched or cut during a windscreen replacement or a dash camera installation
- Water tracking down from a leaking windscreen seal into the sensor connector
- Sensor obstructed by an accessory, sun visor or a mounted device
- Sensor internally failed or knocked loose from its bracket
- Open circuit, short to ground or short to battery on the supply or signal line
- Poor ground for the alarm module
- Aftermarket security system spliced into the factory sensor wiring
How to fix it
- Confirm the manufacturer definition and the front sensor location for this vehicle
- Read the sensor status in live data and wave a hand in front of it to see if it reacts
- Unplug the connector, inspect for water, corrosion and bent terminals, then refit it firmly
- Check for pinched or cut wires around the mirror mount and the A pillar
- Look for windscreen leaks that could feed water into the connector
- Measure supply, ground and signal at the sensor connector
- Swap the front and rear sensors if identical to see whether the fault follows the part
- Repair the wiring or replace the sensor, then arm the system and verify interior detection without false triggers