U0129
Lost Communication with Brake System Control Module
The Brake System Control Module has stopped transmitting on the vehicle data bus, so other control units no longer receive its messages. ABS and electronic brake functions are lost; base braking remains but the assists are off. Typical causes are loss of power or ground at the module, an open or shorted CAN/LIN bus wire, a connector fault, or an internal module failure.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: hard
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Common causes
Ordered by how often each turns out to be the cause.
- Brake module unpoweredCommon
- CAN wiring to brake module brokenPossible
- Gateway module routing CAN messages incorrectlyLess likely
Parts involved
- Brake System Control Module
- CAN Bus Wiring
U0129 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.