U0137 Code Symptoms, Causes, Diagnosis & Repair Guide and Steps

U0137 Code: Symptoms, Causes, and How to Fix It

Drivers who tow a trailer often notice a warning light on the instrument panel that is dedicated to trailer‑brake monitoring. The light may flash or stay solid, and the trailer’s brake‑light circuit may remain active even when the vehicle’s brakes are not applied. In some cases the trailer’s electronic brake‑control system (e.g., trailer ABS) will not engage, resulting in reduced stopping power for the trailer. These are the hallmark signs of the U0137 code: lost communication with the Trailer Brake Control Module.

The problem is confined to the trailer‑brake network; engine performance, fuel delivery, and interior body functions remain unaffected. Because the vehicle’s CAN bus cannot exchange data with the TBCM, any function that depends on that data—brake‑light activation, trailer‑ABS modulation, and electronic brake‑controller commands—fails or behaves erratically.

Symptoms

• Trailer‑brake‑light warning illumination on the dash, often accompanied by a “Trailer Brake” message.
• Continuous trailer brake‑light activation even when the service brakes are released.
• Reduced or absent trailer braking when the vehicle’s brakes are applied, especially noticeable on steep descents.
• Trailer ABS or electronic brake‑controller indicator may appear, indicating loss of data from the TBCM.
• No change in engine, transmission, or interior functions, confirming the issue is isolated to the trailer‑brake system.

Why This Happens

Corroded or Damaged CAN‑Bus Wiring

The CAN bus carries high‑speed data between the vehicle’s control modules and the TBCM. Exposure to road salt, moisture, or physical abrasion can corrode the twisted‑pair wires or their connectors. Corrosion introduces resistance and intermittent opens, breaking the communication link and triggering U0137.

Faulty Power Supply to the TBCM

The TBCM requires a stable 12 V supply, typically derived from the vehicle’s lighting circuit. A blown fuse, a loose ground strap, or a failing voltage regulator can cause the module to lose power intermittently, resulting in a loss of communication.

Internal Module Failure

Control modules contain printed‑circuit boards, microprocessors, and memory chips. Over‑temperature events, moisture ingress, or manufacturing defects can damage these components. When the processor can no longer respond to CAN‑bus requests, the vehicle logs U0137.

Software Corruption

Modern TBCMs store calibration data and communication protocols in flash memory. A failed flash write, an incomplete over‑the‑air update, or a voltage spike during programming can corrupt the software, preventing the module from acknowledging bus messages.

Improper Trailer‑Brake Controller Integration

After installing an aftermarket trailer‑brake controller, mismatched wiring or incompatible communication protocols can prevent the vehicle’s TBCM from recognizing the controller, leading to a U0137 condition. While the controller itself may be functional, the loss of communication is still a module‑level issue.

Diagnostic and Repair Procedures

1. Retrieve the Code and Freeze‑Frame Data

– Connect a professional OBD‑II scanner capable of reading manufacturer‑specific codes. Record the exact timestamp, vehicle speed, and brake‑pedal position at the moment the code was set.

1. Visual Inspection of Wiring and Connectors

– Locate the TBCM (often near the rear bumper or under the trailer‑hitch assembly). Examine the CAN‑high and CAN‑low wires, power feed, and ground strap for corrosion, frayed insulation, or loose terminals. Clean any oxidation with a dielectric cleaner and re‑torque connectors to the manufacturer’s spec (typically 12–15 lb‑ft).

1. CAN‑Bus Continuity Test

– Using a multimeter, verify continuity between the vehicle’s CAN‑high and CAN‑low pins on the TBCM connector and the corresponding pins on the main control module (often the BCM or PCM). Resistance should be less than 60 Ω; higher values indicate a break or severe corrosion.

1. Power and Ground Verification

– Measure voltage at the TBCM power pin while the ignition is ON. A stable 12 V ± 0.5 V indicates a healthy supply. Check ground resistance to chassis; it should be under 0.1 Ω.

1. Communication Test with a Scan Tool

– Many advanced scan tools can issue a “CAN‑bus ping” to the TBCM. A successful response confirms that the module is alive and correctly addressed. No response after confirming power and wiring points to internal module failure or software corruption.

1. Re‑programming Attempt

– If the module responds but the code persists, download the latest TBCM firmware from the vehicle manufacturer and flash it using the scan tool. Re‑run the communication test. Successful re‑programming often clears U0137 without hardware replacement.

1. Module Repair vs. Replacement Decision

– If the TBCM shows no response after power/ground verification, internal damage is likely. Minor board‑level repairs (e.g., solder joint reflow) may be possible but are typically temporary. Replacement is usually more reliable, especially for vehicles older than five years.

Cost Overview

• Diagnostic scan and wiring inspection: $80‑$120 labor.
• Module repair (board‑level): $200‑$400 parts and labor, but success is not guaranteed.
• Replacement TBCM (including programming): $600‑$900 for the unit plus $200‑$300 labor.

When Replacement Makes More Sense Than Repair

If the TBCM fails to communicate after confirming power, ground, and wiring integrity, the probability of a lasting fix from a board‑level repair drops below 30 %. Repeated repair attempts increase labor costs and risk re‑introducing faults. For most vehicles, especially those used for regular towing, a fresh, correctly programmed module restores full trailer‑brake functionality with minimal downtime.

Modern control modules are complex and integrated with security and immobilizer systems. That’s why choosing a replacement isn’t only about the hardware—it’s about correct programming and compatibility. Flagship One specializes in VIN‑matched control modules, providing a plug‑and‑drive solution backed by a lifetime warranty. Replacement units vary depending on production date and software version, so the correct module is matched by VIN before programming. This eliminates the guesswork of generic aftermarket parts and ensures seamless integration with the vehicle’s CAN network.

Preventive Maintenance

• Protect Connectors: Apply dielectric grease to all TBCM connectors during routine trailer‑hitch inspections. This repels moisture and reduces corrosion risk.
• Routine Wiring Checks: Every 12 000 mi, inspect the CAN‑bus harness for signs of wear, especially after winter road‑salt exposure. Replace any compromised sections promptly.
• Power‑Supply Monitoring: Verify that the trailer‑brake‑light fuse remains intact and that the grounding strap stays tight. A loose ground can cause intermittent loss of communication.
• Software Updates: When the vehicle manufacturer releases a TBCM firmware update, have it applied during the next scheduled service. Updated software often includes improved error handling for the CAN bus.
• Proper Trailer‑Brake Controller Installation: Follow the vehicle’s service manual for wiring diagrams and torque specifications. Avoid aftermarket adapters that bypass the OEM TBCM unless they are explicitly approved.