U0129 – Lost Communication With Brake System Control Module means the PCM, ABS module, or other controllers cannot establish communication with the Brake System Control Module (BSCM). This module manages hydraulic brake pressure, regenerative braking (on hybrids/EVs), and stability/traction-related braking functions. When communication is lost, the vehicle may show brake warnings, disabled ABS/ESC, or reduced braking performance. In severe cases, the vehicle can enter limp mode or require increased pedal effort. This guide explains what U0129 means, the symptoms, causes, diagnostic steps, and repairs you should expect.
What Does U0129 Mean?
U0129 is a generic OBD-II communication fault set when the BSCM stops communicating over the CAN bus. This module receives wheel speed data, braking commands, stability requests, and regenerative braking inputs (if equipped). When its data stream disappears, other modules may shut down braking assists to prevent unsafe or unpredictable behavior.
U0129 commonly occurs due to wiring faults, low voltage, connector corrosion, or an internal module failure. Because braking performance may be affected, this code should be diagnosed immediately.
Quick Reference
- OBD-II Family: U-Code (Network Communication)
- Scope: Generic
- System: Brake Control / ABS / CAN Bus
- Difficulty Level: Moderate to Advanced
- Estimated Repair Cost: €150–€900+
- Last Updated: 21 November 2025
Real-World Example / Field Notes
An EV owner reported that regenerative braking suddenly stopped working and the brake warning light illuminated. The scan tool showed U0129 in the PCM, ABS, and VDCM. Water intrusion inside the BSCM connector caused intermittent communication loss. Drying the connector, applying dielectric grease, and sealing the grommet restored normal braking and regen operation.
Another case involved a pickup truck with ABS and traction control lights on. U0129 was logged consistently. The root cause was a CAN wiring break near the left rear frame rail where road debris had damaged the harness. Repairing the wiring re-established communication instantly.
Symptoms of U0129 – Lost Communication With Brake System Control Module
- ABS warning light: Anti-lock braking disabled.
- Traction control and ESC lights: Stability systems reduced or disabled.
- Brake warning light: Vehicle may rely solely on hydraulic braking.
- Loss of regenerative braking: Common on hybrids and EVs.
- Harder brake pedal feel: Pressure control unavailable.
- No communication with the ABS/BSCM module: Scan tool cannot connect.
- Longer stopping distances: Safety systems offline.
Common Causes of U0129
Most Common Causes
- Loss of power or ground at the BSCM
- Corroded connectors near ABS pump or hydraulic unit
- CAN wiring damage (shorts, opens, high resistance)
- Failed Brake System Control Module
- Low voltage or charging system failure
Less Common Causes
- Rodent damage to frame-rail wiring
- Aftermarket trailer brake wiring interfering with CAN
- Water intrusion from wheel wells or underbody splashing
- Failed ABS pump drawing excessive current
- Poor chassis grounding near frame or battery
Diagnosis: Step-by-Step Guide
U0129 diagnosis focuses on BSCM power, CAN communication integrity, and physical wiring inspection, especially near the ABS hydraulic unit.
Tools You’ll Need
Advanced scan tool, multimeter, wiring diagrams, backprobe kit, CAN breakout box, oscilloscope (recommended), test light, and battery/alternator tester.
- Scan all modules: Confirm U0129 across modules and verify the BSCM does not respond.
- Inspect BSCM location: Usually attached to the ABS pump or mounted on the frame rail.
- Check power and ground: Ensure proper voltage (battery-level) and clean grounds.
- Inspect connectors: Look for corrosion, moisture, road salt, or bent terminals.
- Test CAN-H and CAN-L voltages: Expect ~3.5V (H) and ~1.5V (L). Incorrect readings indicate wiring faults.
- Measure network termination: CAN-H to CAN-L should be ~60 ohms ignition off.
- Check harness routing: Inspect frame rails, wheel wells, and areas exposed to debris.
- Disconnect nearby modules: A failing module may overload the network, blocking BSCM communication.
- Perform continuity testing: Verify the CAN line integrity from the BSCM to the VDCM/PCM.
- Oscilloscope waveform test: Look for noise, flatlining, or distorted differential signals.
Pro Tip: On many vehicles, the BSCM connector faces the road and collects dirt, water, and salt. Cleaning and sealing this connector often restores communication without major component replacement.
Possible Fixes & Repair Costs
- Clean or replace corroded BSCM connector: €20–€80
- Repair damaged CAN wiring: €60–€200
- Replace BSCM ground straps or fix poor grounding: €20–€80
- Replace Brake System Control Module: €250–€900+
- Repair hydraulic ABS pump wiring: €80–€200
- Charging system repair: €120–€400
Can I Still Drive With U0129?
You can usually drive, but braking performance may be reduced. ABS, traction control, and stability control may all be disabled. On hybrids and EVs, regenerative braking may stop working, increasing wear on the mechanical brakes. If the brake warning light is red or the pedal feels significantly harder, it may not be safe to continue driving.
Related Codes
- U0151 – Instrument Panel Communication Fault
- U0141 – Steering System Network Communication Fault
- U0121 – Lost Communication With Anti-Lock Brake System (ABS) Control Module
- U0102 – Lost Communication With Transfer Case Control Module
- U0101 – Lost Communication With TCM
- U0100 – Lost Communication With ECM/PCM "A"
- U0195 – Lost Communication With Audio AMP
- U0194 – Lost Communication With Audio Display
- U0188 – Lost Communication With Video Display
- U0185 – Lost Communication With TV/Telematics Unit
Key Takeaways
- U0129 means the Brake System Control Module is offline or not communicating.
- Safety systems like ABS, ESC, and traction control may be disabled.
- Common causes include corrosion, wiring faults, and low voltage.
- Repairing wiring or sealing connectors often resolves the issue.
FAQ
Is the BSCM the same as the ABS module?
In many vehicles, yes — the BSCM is integrated with the ABS hydraulic unit. In others, it is a separate controller.
Can U0129 be caused by a weak battery?
Absolutely. Low voltage or alternator ripple can cause modules to drop off the CAN network.
Does U0129 affect braking performance?
Yes. ABS, traction control, and regen braking may be disabled, increasing stopping distances.
Is water intrusion common for U0129?
Very common, especially in regions with road salt. The module and connectors are often exposed to the elements.
Should I replace the module right away?
No. Always check wiring, connectors, grounds, and voltage before replacing expensive modules.