U0129 U0xxx · Network (U) High severity
Lost Communication with Brake System Control Module
Communication with the brake control module is lost. On vehicles with integrated braking (regenerative or brake-by-wire) the effect is more severe than on conventional systems.
Severity guidance: High — stop driving and diagnose now; risk of further damage
Symptoms you will notice
- Brake warning lamp
- Reduced braking assistance
- Regenerative braking unavailable
Common causes
Ordered from most to least likely — start at the top.
- 1 Power or earth fault at the brake module
- 2 CAN wiring fault
- 3 Failed module
- 4 Water ingress
How to diagnose it
Cheapest and least invasive first — most of these codes are not a parts problem.
- ✓ Check fuses and supply at the brake module.
- ✓ Verify CAN signals at the connector.
- ✓ Inspect for water ingress.
- ✓ Read the brake module directly if partially reachable.
Repair options
- Repair supply or bus wiring
- Replace the brake control module
- Seal against water ingress
U0129 — frequently asked questions
Is it safe to drive? +
Treat as a recovery job. Integrated braking systems lose much more than conventional ABS when they go off-line.
Does regen stop? +
Yes — the vehicle falls back to friction braking only, which changes pedal feel and range.
Confirm the diagnosis before you buy parts
Send us the code, freeze-frame data and the vehicle — our engineers will tell you what to test first.