OBD-II · Network and communication (U)
U0129
Lost communication with brake system control module
What it means
Another module has stopped receiving messages from the brake system control unit (on newer cars this can be an electric brake booster or brake-by-wire unit).
Symptoms
- Brake warning light
- Brake pedal feel may change
- ABS/ESP and driver-assist functions disabled
Likely causes, most likely first
| No. | Cause |
|---|---|
| 01 | weak 12V battery or low voltage |
| 02 | blown brake system fuse |
| 03 | corroded connector at the brake control unit |
| 04 | CAN wiring fault |
| 05 | failed brake system control module |
What to check, step by step
- 01Stop safely and arrange recovery if pedal feel or braking changes, a red brake warning remains on with the parking brake released, or a stop message appears
- 02Identify the reporting module and whether the code is current or historical; scan the brake module and all related modules
- 03Check the 12V battery and specified fuses
- 04Inspect connectors and test power, ground and CAN at the brake module using the OEM procedure