U0137: Lost Communication With Trailer Brake Control Module
What to do
Do not tow anything until this is fixed. That is the whole of the advice, and it needs saying because nothing about driving will warn you.
This code means the system that applies your trailer's brakes has stopped working. The trailer now has no brakes — your vehicle is stopping both on its own.
Your own brakes feel completely normal, which is the problem. You can hitch up, drive for an hour and notice nothing at all. You find out at the first emergency stop, or on the first long hill — the trailer pushes from behind, the stopping distance is far longer than you expect, and your brakes overheat doing work they were never sized for.
In most countries it is also illegal, and an insurance problem, to tow a trailer above a modest weight without working brakes — whatever the reason.
The breakaway cable does not cover this. It only applies the trailer brakes if the trailer comes off; it does nothing during normal driving.
One free test worth doing: unplug the trailer and see whether the fault clears. The socket at the back of the vehicle sits low, in the spray and salt, is used rarely and cleaned never — corroded pins and water in the socket are far commoner than a failed module, and that test tells the garage immediately whether to look at the vehicle or the trailer.
What this code means
The trailer brake control module has stopped being heard from. The general reasoning is at U0100, and U0438 covers the same module reporting implausibly instead.
This page has one job: do not tow while this is stored.
An integrated trailer brake controller applies a trailer's electric brakes in proportion to the tow vehicle's own braking. When the module goes silent, the trailer receives no signal at all — so a braked trailer becomes an unbraked one, and the vehicle in front of it is doing all the work.
The reason this needs saying plainly is that nothing tells the driver. The tow vehicle's brakes are unaffected and feel entirely normal at ordinary speeds and gentle stops. A driver can hitch up, pull away, drive for an hour and notice nothing. The discovery happens at the first emergency stop, or on the first long descent, which is exactly the wrong moment to find out.
What actually changes:
- Stopping distance increases substantially — a loaded trailer can weigh as much as the vehicle towing it, and the brakes sized for one are now stopping both.
- On a descent the trailer pushes, which is what unsettles the outfit and starts a sway.
- The tow vehicle's brakes overheat doing work they were not specified for, and brake fade on a long hill is a real outcome rather than a theoretical one.
And it is very often a legal matter as well. In most jurisdictions a trailer above a modest weight must have working brakes, and towing without them is an offence and an insurance problem irrespective of what caused it.
The breakaway cable is not a substitute. It applies the trailer's brakes fully if the trailer detaches; it does nothing during normal braking.
Where the fault usually is, and it is rarely the module. The trailer socket and its wiring live at the very back of the vehicle, low down, in the spray and the salt, and are used rarely and cleaned never. Corroded pins, a socket full of water, and a towing loom spliced into the harness rather than fitted through the vehicle's proper interface account for most of these. Unplug the trailer and see whether the code changes — that free test separates the vehicle from the trailer immediately.
Likely causes
Start here
- Corroded or loose connector · easy
- Water ingress into the module connector · moderate
- Poor wiring splice in an aftermarket installation · moderate
- Damaged wiring harness · moderate
Less common
- Blown fuse or failed supply · easy
- Damaged data bus wiring · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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