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U0129 — Lost Communication Brake System Control Module

Generic OBD-II · all makesNetwork / communication

U0129 is set when the powertrain control module (PCM) detects a loss of communication with the brake system control module (BSCM) over the Controller Area Network (CAN) bus. This usually happens after a preset period during which no valid CAN messages are received from the brake module, causing the PCM to store the fault code and illuminate the brake warning light. The underlying problem can be a wiring/connector issue, a power/ground failure to the BSCM, a faulty module itself, or, rarely, a problem with the PCM's network handling.

Symptoms

What causes it — most likely first

1. Faulty or dead Brake System Control Module (BSCM)

The module itself may have failed internally, preventing it from sending CAN messages.

2. Loss of power or ground to the BSCM

A blown fuse, bad relay, or corroded ground can leave the module without voltage, causing it to go silent.

3. CAN bus wiring or connector damage

Open, short, or high‑resistance wiring, or a loose connector, will interrupt CAN communication.

4. Network interference or other module faults

A fault in another CAN node can corrupt messages, leading the PCM to detect a loss of communication with the brake module.

5. Faulty PCM/ECU (rare)

The engine control module can misread or fail to process CAN messages, but this is an uncommon cause and should be investigated only after all other possibilities are ruled out.

How it's diagnosed

  1. Verify the U0129 code with a scan tool and note any additional U‑codes that may point to network or module issues.
  2. Inspect the BSCM fuse and power supply; confirm proper voltage and ground at the module harness.
  3. Visually examine the CAN bus wiring and connectors for damage, corrosion, or looseness.
  4. Perform a CAN bus resistance test; a healthy network typically shows about 60 Ω differential resistance.
  5. Swap or bench‑test the BSCM if electrical checks are normal to determine if the module itself has failed.
  6. As a final step, check the PCM for faults and, if needed, reprogram or replace it only after all other causes are eliminated.

Code facts & related codes

SystemNetwork / communication
StandardizationSAE generic — identical on every make
Related codesU0120 — Lost Communication Starter Generator CU0121 — Lost Communication Anti Lock Brake SysU0122 — Lost Communication Vehicle Dynamics CoU0123 — Lost Communication Yaw Rate Sensor ModU0124 — Lost Communication Lateral AcceleratioU0125 — Lost Communication Multi Axis Accelera

Frequently asked

Can a bad PCM cause U0129?

While the PCM is the device that detects the loss of communication, it is rarely the root cause. Most cases stem from the brake module, its power/ground, or the CAN wiring. Replacing the PCM should be a last resort after all other possibilities have been eliminated.

Will my car still be drivable with U0129?

The vehicle may still run, but you will likely lose ABS, electronic brake‑force distribution, and brake assist functions. Driving with this code is unsafe, especially in conditions that require full braking performance.

Is it safe to clear the code and ignore it?

Clearing the code without fixing the underlying problem will cause the warning light to return and may leave critical braking features disabled. The root cause should be diagnosed and repaired before resetting.

How much does it cost to fix U0129?

Costs vary widely based on the cause; a simple fix like a blown fuse or wiring repair is inexpensive, while a replacement brake control module can be costly. Always obtain a proper diagnosis before assuming the most expensive repair.