U0137 - Lost communication with Trailer Brake Control Module
System
Network & Communication (CAN-bus)
Technical Description
The control module has detected a fault in the Data Bus Communication (Trailer Brake Control Module) (U0137). Node involved: Trailer Module (lighting / brake). Carried over the vehicle data bus. Communication with the module has dropped out entirely — the controller no longer receives any frames from it on the data bus.
Multimeter Test Points & Nominal Values
| Test Point / Component | Measurement | Nominal Target Range | Test Condition |
|---|---|---|---|
| Check for "Trailer Module (lighting / brake)" (U0137) (Action ()) | Mätning | Measure ignition supply and ground right at the module connector, then check CAN continuity from the module pins back to the gateway. | Vehicle data bus |
| CAN Termination (OBD pin 6 ↔ 14) | Resistance (Ω) | 60 Ω (two 120 Ω in parallel) | Battery disconnected, ignition OFF |
| CAN-H / CAN-L Bias Voltage | Voltage (V) | CAN-H ≈ 2.5 - 3.5 V, CAN-L ≈ 1.5 - 2.5 V (recessive ≈ 2.5 V both) | Ignition ON, bus active |
| Trailer Brake Control Module – Power and Ground | Voltage (V) | Supply 11.8 - 14.4 V / ground drop < 0.1 V | At module connector under load |
Vehicle data bus
Battery disconnected, ignition OFF
Ignition ON, bus active
At module connector under load
Oscilloscope Waveform Analysis
10 µs/div, 1 V/divPossible Causes
The module has lost power or ground, or the bus stub feeding it is broken
Module location:Trailer Module (lighting / brake) — in the rear corner of the boot at the tow socket — connector corrosion. Check connector, power and ground right there.
Bus segment:look up topology, speed and termination in the wiring diagram.
Loose or corroded splice where the bus stub to the Trailer Module (lighting / brake) branches off the backbone.
An internal fault in the Trailer Module (lighting / brake) or its bus transceiver — confirm with a known-good module only after wiring and power are ruled out.
Symptoms
Recommended Solution
Measure ignition supply and ground right at the module connector, then check CAN continuity from the module pins back to the gateway.
Physically get to the Trailer Module (lighting / brake) (in the rear corner of the boot at the tow socket — connector corrosion), unplug the connector and inspect the pins for corrosion, spreading and moisture.
Measure ignition supply and ground at the Trailer Module (lighting / brake) under load (voltage drop < 0.1 V) and check the module dedicated fuse.
Measure bus resistance across CAN-H/CAN-L, ignition off: 60 Ω normal, ~120 Ω = a terminator/branch missing, <10 Ω = conductor-to-conductor short.
Read DTCs in EVERY module:a single dead node points at this module or its stub, while U-codes in several modules point at the backbone or gateway.
Mechanic's Tip
If several modules drop together the fault is on the backbone or gateway; if only this one drops, the fault is local to the module.