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U0137 Lost Communication with Trailer Brake Control Module

OBD-II Trouble Code Technical Description

Article by
Randy Worner
ASE Certified Master Technician

Lost Communication with Trailer Brake Control Module

What does that mean?

This is a generic communication system diagnostic trouble code that applies to most vehicle makes and models.

This code means that the Trailer Brake Control Module (TBCM) and other control modules on the vehicle are not talking to each other. The circuit most often used to communicate with is known as Controller Area Network bus communications, or simply put, CAN bus.

Without this CAN bus, control modules cannot exchange information, and your scan tool may not be able to get information from the vehicle, depending on which circuit is affected.

The TBCM receives input from a variety of sensors, some hardwired directly to it, some are sent over the bus communications system. These inputs allow the module to control the vehicle when a trailer is attached to it. When the base brakes are energized, it prevents the trailer from overtaking the vehicle by alternately applying the brakes at different corners of the vehicle. Typically, this system is found on SUVs and light duty to medium duty trucks but can be utilized as the manufacturer sees fit for other alternative applications.

Troubleshooting steps may vary depending upon manufacturer, type of communications system, number of wires and wire colors in the communication system.

Severity & Symptoms

Severity in this case depends upon the system. Because the parking brakes are integrated into the base brakes / rear calipers, care should always be taken with this system. Also, safety is a concern during servicing these systems as well. ALWAYS consult service information prior to disassembly/diagnosing these systems.

Symptoms of a U0137 engine code may include:


Typically the causes for this code to set are:

Diagnostic and Repair Procedures

A good starting point is always to check for technical service bulletins (TSB) for your particular vehicle. Your issue may be a known issue with a known fix put out by the manufacturer and can save you time and money during diagnosis.

If your scan tool can access fault codes and the only one you retrieve from other modules is the U0137, try to access the trailer brake control module. If you can access codes from the TBCM module, then the U0137 code is either intermittent or a memory code. If unable to access codes for the TBCM module, then the U0137 code that the other modules are setting is active, and the problem is there now.

The most common failure is loss of power or ground to the TBCM module.

Check all fuses that power up the TBCM module on this vehicle. Check all grounds for the TBCM. Locate where the ground attaching points are on the vehicle and make sure that these connections are clean and tight. If you have to, take them off, get a small wire bristle brush and baking soda/water solution and clean each one, both the connector and where it connects.

If any repairs were made, clear the diagnostic trouble codes from memory, and see if the U0137 code returns or if you are able to communicate with the TBCM module. If the code does not return or communication is re-established, then the fuses/connections were most likely your problem.

If the code returns, locate the CAN C bus communication connections on your particular vehicle, most importantly the TBCM module connector. Disconnect the negative battery cable before unplugging the connector at the TBCM control module. Once located, visually inspect the connectors and wiring. Look for scraping, rubbing, bare wires, burn spots or melted plastic. Pull the connectors apart and carefully inspect the terminals (the metal parts) inside the connectors. See if they look burned or have a green tint indicating corrosion. Use electrical contact cleaner and a plastic bristle brush if cleaning of the terminals is needed. Let dry and apply electrical grease where the terminals contact.

Before connecting the connectors back to the TBCM module, make these few voltage checks. You will need to have access to a digital volt-ohmmeter (DVOM). Verify that you have power and ground at the TBCM module. Gain access to a wiring diagram and determine where the main powers and grounds come into the TBCM module. Reconnect the battery before continuing, with the TBCM module still disconnected. Connect the red lead of your voltmeter to each B+ (battery voltage) supply coming into the TBCM module connector and the black lead of your voltmeter to a good ground (if not sure, battery negative always works). You see a reading of battery voltage. Verify that you have good grounds as well. Hook the red lead of your voltmeter to battery positive (B+) and the black lead to each ground circuit. Once again you should see battery voltage at each connection. If not, repair the power or ground circuit problem.

Next, check the two communication circuits. Locate the CAN C+ (or HSCAN + circuit) and CAN C- (or HSCAN – circuit). With the black lead of your voltmeter connected to a good ground, connect the red lead to CAN C+. With the Key On, Engine Off, you should see about 2.6 volts and fluctuating slightly. Next, connect the red voltmeter lead to the CAN C- circuit. You should see approximately 2.4 volts and fluctuating slightly. Other manufacturers show CAN C- at approximately .5 volts and fluctuating Key On Engine Off. Check the specifications for your manufacturer.

If all tests have passed and communication is still not possible, or you were unable to clear the U0137 fault code, the only thing left that can be done is to seek assistance from a trained automotive diagnostician, as this would indicate a failed TBCM module. Most of these TBCM modules must be programmed, or calibrated to the vehicle in order to be installed correctly.

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