U0129 - LOSS OF COMMUNICATION WITH BRAKE SYSTEM CONTROL MODULE (BSCM)

The Controller Area Network (CAN) is the circuit generally used by control modules to communicate with each other. It is through this circuit that information and various vehicle operating commands can be exchanged. Also, it is through the CAN bus that information can be obtained from the car with the use of a scanner.

The BSCM (Brake System Control Module) is a controller that receives information from different sensors, through which the base brakes can be activated, although it is also used as a combined base/anti-brake system. It is worth mentioning that this system is very common to see in hybrid cars, but certain manufacturers consider it suitable for various applications.

If the Brake System Control Module (BSCM) fails to communicate with another control module that requires such information, a DTC will be set that serves as a warning of the situation. In the case of the U0129 OBDII code, it indicates that the communication from that controller could not be established.

• Check Engine light on.
• The Brake system exhibits performance instabilities.

The reasons for the U0129 OBD2 diagnostic code storage occurring are:

• The CAN bus communication protocol may have deficiencies in its wiring, thus causing an open circuit or short to ground.
• The Brake System Control Module may have a short ground or power supply.
• The Brake System Control Module may be damaged.

The steps to follow to correct the U0129 OBDII fault code are:

• Start the diagnosis by consulting the TSBs.
• Check if other DTCs are stored, especially those that refer to failures of a particular controller. If this is the case, diagnose those codes.
• Check the CAN high and CAN low lines for malfunction caused by a short or open circuit. Look for wiring damage or poor connections. Repair it as necessary.
• Measure the ground and power circuit voltages of the Brake System Control Module. The values obtained from these tests should match those given in the manufacturer's specifications, otherwise, corrections are necessary.