What it means
A receiving module detected invalid data from the trailer brake control module. The code does not identify a specific failed part or establish that trailer braking is affected.
At a glance
- Usually lights the warning light of the system that lost communication
How it works
The trailer brake control module communicates the status of the vehicle's integrated trailer braking system to other modules. U0438 sets when a receiving module rejects data from that trailer brake controller as invalid; the code alone does not prove that trailer brakes have failed. A power, ground, connector, network, or controller fault can interrupt valid status messages.
Common causes
Most likely first.
- Invalid data from the trailer brake control module
- Trailer brake module power, ground, connector, or network fault
- Trailer brake module fault
Symptoms
- Trailer brake warning or message may appear
- Integrated trailer braking may be unavailable or limited
- No noticeable symptoms may occur
How to diagnose it
- Scan all modules and save U0438, trailer brake codes, and freeze-frame data to identify which module rejected the trailer controller's messages.
- Check the dashboard and trailer brake controller for warnings, and determine whether integrated trailer braking responds as expected before towing.
- Inspect the trailer brake module, vehicle and trailer connectors, power supply, ground, and CAN wiring for corrosion, damage, or loose pins.
- Check live trailer brake status and communication data with a scan tool, including whether the fault occurs with the trailer connected or disconnected.
- Test module power, ground, and CAN communication to the manufacturer's specification before condemning the trailer brake controller.
Common fixes
- Clean and secure corroded or loose trailer brake module or connector contacts.
- Repair a damaged power, ground, or CAN wire to the trailer brake controller.
- Repair a faulty trailer connector or wiring connection identified during testing.
- Correct module configuration or software if compatibility is the cause.
- Replace the trailer brake control module only after power, ground, and network checks pass.
Can I keep driving?
Do not tow until you have confirmed the trailer's braking system works; integrated trailer braking may be limited or unavailable even if the vehicle drives normally. If a trailer is attached and its brakes are not responding, stop safely and do not continue towing.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.