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DTC codes / U01xx · Lost communication with a control module / U0137

U0137Lost Communication with Trailer Brake Control Module

U0137 is a network communication code indicating that the vehicle's main control modules have lost contact with the trailer brake control module over the controller area network (CAN) bus. The computer has detected that expected periodic messages from the trailer brake module are missing, so it flags the module as offline and may disable trailer braking functions.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaLost communication with a control module

Symptoms you may notice

  • Illumination of the anti-lock brake system (ABS) or traction control warning light
  • A message on the driver information center stating 'Trailer Brake System Fault' or similar
  • No trailer brake output, meaning electric trailer brakes do not engage when the tow vehicle brakes are applied
  • Intermittent operation of trailer brakes, especially over bumps or in wet conditions
  • Often no symptoms besides the warning light when no trailer is connected
  • Other unrelated warning lights may appear if the module shares a bus segment with other systems

Common causes, most likely first

  1. Corroded, damaged, or disconnected electrical connector at the trailer brake control module, often due to water intrusion or road debris
  2. Broken, chafed, or open circuit in the CAN bus wiring between the trailer brake module and the main harness
  3. Failed trailer brake control module, either internally shorted or unresponsive
  4. Blown fuse or poor ground connection supplying power to the trailer brake control module
  5. Aftermarket accessories tapped into the CAN bus or trailer wiring causing signal interference
  6. Software incompatibility or failed module configuration after battery replacement or trailer brake controller installation

How to diagnose and fix it

  1. Inspect the trailer brake control module connector for corrosion, water intrusion, or loose pins, and clean or repair as needed
  2. Check all fuses and grounds related to the trailer brake control module and trailer wiring harness
  3. Perform a visual inspection of the CAN bus wiring for chafing, breaks, or poor splices, especially near the trailer hitch and under the vehicle
  4. Use a scan tool to check for communication with the trailer brake control module and perform a network integrity test
  5. Measure CAN bus voltage and resistance at the module connector to confirm signal presence and rule out wiring faults
  6. If wiring and power are confirmed good, substitute a known-good trailer brake control module to verify the fault
  7. After repair, clear codes and verify communication is restored and trailer brake function operates correctly

Trailer brake module communication pitfalls

  • A common oversight is assuming the trailer brake module itself is faulty when the real problem is a corroded connector behind the bumper that only acts up in wet weather.
  • The code can be set by a completely disconnected trailer brake module on vehicles that use a plug-and-play harness, so check for a missing module before diagnosing wiring.
  • Aftermarket trailer brake controllers or wiring taps can back-feed the CAN bus and cause U0137 even when the factory module is healthy.
  • Some vehicles require the trailer brake module to be configured or learned after replacement, and skipping this step leaves the code stored even with good hardware.
  • U0137 often appears alongside U0100 or U0121 when the bus is shorted, so do not replace multiple modules until the shared wiring is ruled out.

Can I keep driving with U0137?

You can usually drive the vehicle without a trailer, but trailer braking will be disabled, so do not tow until the fault is repaired. If the warning light is accompanied by ABS or traction control warnings, have the system inspected promptly because those safety systems may also be affected.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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