U0129 Lost Communication With Brake Control Module (BSCM)
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OBD-II Trouble Code - U0129 Data Sheet
Lost Communication With Brake Control Module (BSCM)
This code indicates that the Brake Control Module (BSCM) and other control modules on the vehicle are not communicating with each other. The scheme most commonly used for communication is known as Controller Area Network bus communication or, more simply, CAN bus.
What does DTC U0129 mean?
This is a generic communication system diagnostic trouble code that applies to most makes and models of vehicles.
This code means that the brake control module (BSCM) and other control modules on the vehicle are not communicating with each other. The circuitry most commonly used for communication is known as Controller Area Bus communication, or simply the CAN bus.
Without this CAN bus, control modules cannot communicate and your scan tool may not receive information from the vehicle, depending on which circuit is involved.
The BSCM receives input data from various sensors, some of which are directly connected to it and some of which are sent over a bus communication system. These inputs allow the module to control the activation of the basic brakes, but can also be used as a combined basic braking / anti-braking system. Typically this system is used on hybrid vehicles, but can be used for other alternative applications if the manufacturer deems it appropriate.
Troubleshooting steps may vary depending on the manufacturer, the type of communication system, the number of wires, and the colors of the wires in the communication system.
Severity and symptoms
The severity in this case depends on the system. Normally, in the event of a brake control module failure, only the base brakes / no anti-lock braking system / no traction control system / no advanced brake warning system are redundant.
Symptoms of a U0129 engine code may include:
- Red brake warning light is on
- Deterioration of the braking system
Causes of error U0129
Usually the reason for installing this code is:
- Open in CAN + bus circuit
- Open in the CAN bus - electrical circuit
- Short circuit to power in any CAN bus circuit
- Short to ground in any CAN bus circuit
- Loss of power or ground to the BSCM - the most common
- Rarely - the control module is faulty
Diagnostic and repair procedures
A good starting point is always to check the Technical Service Bulletins (TSB) for your particular vehicle. Your problem may be a known issue with a known manufacturer-released fix and could save you time and money while troubleshooting.
If your scan tool can access trouble codes and the only code you are pulling from other modules is U0129, try accessing the BSCM. If you can access the codes from the BSCM, then code U0129 is either intermittent or a memory code. If the codes for the BSCM module cannot be accessed, then code U0129 that other modules set is active and the problem already exists.
The most common failure is a loss of power or ground to the BSCM.
Check all fuses supplying the BSCM on this vehicle. Check all grounds for the BSCM. Locate ground anchorage points on the vehicle and make sure these connections are clean and secure. If necessary, remove them, take a small wire bristle brush and baking soda / water solution and clean each one, both the connector and the place where it connects.
If any repairs have been made, clear the DTCs from memory and see if U0129 returns or you can contact the BSCM. If no code is returned or communication is restored, the problem is most likely a fuse / connection issue.
If the code returns, look for the CAN C bus connections on your specific vehicle, especially the BSCM connector. Disconnect the negative battery cable before disconnecting the connector on the BSCM. Once detected, visually inspect the connectors and wiring. Look for scratches, scuffs, exposed wires, burn marks, or molten plastic. Disconnect the connectors and carefully inspect the terminals (metal parts) inside the connectors. See if they look burnt or have a green tint indicating corrosion. If you need to clean the terminals, use an electrical contact cleaner and a plastic bristle brush. Allow to dry and apply electrical grease where the terminals touch.
Perform these few voltage checks before connecting the connectors back to the BSCM. You will need access to a digital volt/ohmmeter (DVOM). Make sure you have power and ground on the BSCM. Access the wiring diagram and determine where the main power and ground supplies enter the BSCM. Reconnect the battery before continuing with the BSCM still disabled. Connect the red lead of your voltmeter to each B+ (battery voltage) power supply plugged into the BSCM connector, and the black lead of your voltmeter to a good ground (if unsure, battery negative always works). You see the battery voltage reading. Make sure you have a good reason. Connect the red lead of the voltmeter to the battery positive (B+) and the black lead to each ground circuit. Once again, you should see the battery voltage every time you connect. If not, repair the power or ground circuit.
Then check the two communication circuits. Locate CAN C+ (or HSCAN+) and CAN C- (or HSCAN - circuit). With the black wire of the voltmeter connected to a good ground, connect the red wire to CAN C+. With the key on and engine off, you should see about 2.6 volts with little fluctuation. Then connect the red wire of the voltmeter to the CAN C- circuit. You should see about 2.4 volts with little fluctuation. Other manufacturers show CAN C- at about 5V and an oscillating key with the engine off. Check your manufacturer's specifications.
If all tests pass and communication is still not possible, or you were unable to reset DTC U0129, the only thing to do is to seek help from a trained automotive diagnostician, as this will indicate a faulty BSCM. Most of these BSCMs must be programmed or calibrated for the vehicle in order to be properly installed.
Code U0129 Brand specific information
- U0129 Buick lost communication with the brake control module
- U0129 Cadillac lost communication with the brake control module
- U0129 Chevrolet lost communication with the brake control module
- U0129 Ford lost communication with the brake control module
- U0129 GMC lost communication with the brake control module
- U0129 Lincoln lost communication with the brake control module
- U0129 Mercury lost communication with the brake control module
Need more help with code u0129?
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NOTE. This information is provided for informational purposes only. It is not intended to be used as a repair recommendation and we are not responsible for any action you take on any vehicle. All information on this site is protected by copyright.
One comment
Hasan
i can't communicate with the ABS module