OBD-II trouble code
U0137: Lost Communication with Trailer Brake Control Module
Another module on the vehicle network can no longer hear from the integrated trailer brake controller. The truck itself drives normally, but the electric brakes on a towed trailer may not apply — which matters a great deal the moment you hitch something up.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $0 – $900
- DIY difficulty
- Intermediate DIY
What does U0137 mean?
Many pickups and full-size SUVs come with a factory-integrated trailer brake controller. Its job is to apply the electric brakes on a towed trailer in proportion to how hard the tow vehicle is braking, and to let the driver apply the trailer brakes manually with a lever on the dash. To do that it has to know what the rest of the vehicle is doing — brake pedal application, deceleration rate, vehicle speed, ABS activity — and it gets all of that as messages on the vehicle's communication network, usually a CAN bus. U0137 sets when another module stops receiving messages from that controller.
The important thing to understand is what this code does and does not affect. The tow vehicle's own brakes are entirely mechanical and hydraulic and are unaffected — the truck stops exactly as it always has. What is lost is the tow vehicle's ability to command the trailer's electric brakes. Unhitched, the practical consequence is a warning light and a message on the dash. Hitched to a loaded trailer, the consequence is that the tow vehicle is now stopping the trailer's mass through its own brakes alone, which lengthens stopping distances substantially and can induce sway or jackknifing under hard braking or on a descent. That is why this code, which is harmless on an empty truck, is treated as serious.
The causes are the ordinary lost-communication faults rather than anything exotic. A blown fuse or a lost ground leaves the module unpowered and therefore silent. Damaged, chafed, or corroded communication wiring breaks the path. The controller's connector can corrode or back out. The module itself can fail internally. Another module on the same bus can fault in a way that disrupts network traffic for everyone. And low system voltage from a weak battery causes intermittent dropouts that come and go with no obvious pattern.
One cause is worth calling out because it accounts for a large share of these codes in the real world: the trailer wiring connector at the rear bumper. It sits low, is exposed to road salt, water, and physical damage, and gets plugged and unplugged repeatedly. A shorted or corroded trailer connector can pull down the brake output circuit hard enough to damage the controller's driver or trip its protection, and a short introduced by a faulty trailer can take out a perfectly good controller. Any diagnosis of U0137 on a truck that tows should include inspecting the seven-way connector and, if a trailer is involved, testing with the trailer unplugged to determine which side of the connection actually has the fault.
Common causes
- Blown fuse feeding the trailer brake control module
- Corroded or loose ground for the trailer brake controller
- Damaged, chafed, or corroded communication wiring to the controller
- Corroded or backed-out terminals at the controller connector
- Corroded, shorted, or water-damaged seven-way trailer connector at the rear bumper
- Short circuit in the trailer's own wiring pulling down the brake output and damaging the controller driver
- Failed internal communication circuitry in the trailer brake control module
- Another module on the bus disrupting network communication for everyone
- Low 12-volt system voltage from a weak or failing battery
- Water intrusion into the module, which is often mounted low in the dash or under the steering column
- Aftermarket trailer wiring or a brake controller spliced into the factory harness
Symptoms
- Trailer brake warning light or a "trailer brake system" message on the dash
- Trailer brake gain and manual lever settings unavailable or not adjustable
- Electric trailer brakes not applying when towing
- Noticeably longer stopping distances with a trailer attached
- Trailer brake display missing from the instrument cluster or information center
- Check engine light may also be on
- Scan tool unable to communicate with the trailer brake control module
- Symptoms that come and go, which usually indicates a marginal connection or low voltage
- Other network codes stored alongside U0137
Diagnostic steps
- 1.Scan all modules and record every stored code. If several modules are unreachable rather than just the trailer brake controller, the problem is the network itself, not this module.
- 2.Check the fuses and relays feeding the trailer brake control module before anything else. A blown fuse is the single most common cause and the cheapest to find.
- 3.Inspect the seven-way trailer connector at the rear bumper for corrosion, bent or pushed-back pins, moisture, and evidence of arcing. This connector is exposed to salt and water and fails often.
- 4.If a trailer was attached when the fault appeared, unplug it and retest. A short in the trailer's brake wiring can trip or damage the controller, and testing without the trailer separates the two.
- 5.Locate the module — commonly under the dash near the steering column on full-size trucks — and inspect it and its connector for water intrusion and corrosion.
- 6.Verify power and ground at the module connector with a voltage-drop test under load rather than a simple continuity check.
- 7.Measure the communication bus lines at the module connector and verify continuity back to the rest of the network.
- 8.Load-test the battery and verify charging system output. Low voltage produces intermittent communication loss across multiple modules.
- 9.Inspect the wiring run from the module to the rear of the vehicle for chafing, rodent damage, and prior splices from aftermarket towing accessories.
- 10.If power, ground, bus continuity, and connectors are all good and the module still will not communicate, an internal module fault is the remaining conclusion — replacement typically requires programming to the vehicle.
- 11.After repair, verify by hitching a trailer and confirming the gain adjusts and the manual lever applies the trailer brakes before towing anything loaded.
Repair cost
$0 – $900
A blown fuse or a corroded connector that cleans up is effectively free once found, which is why the low end is zero. Cleaning or replacing a seven-way trailer connector typically runs $40-$200. Repairing a corroded ground or a section of communication wiring generally runs $150-$450. Diagnostic time often adds $100-$200 because network faults require checking multiple modules. Replacing and programming a trailer brake control module is the most expensive outcome at roughly $400-$900 depending on the vehicle, and should be the last step rather than the first.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with module communication / can bus diagnosis preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.