U0138 means the vehicle’s communication network has lost data contact with the All Terrain Control Module, the controller that oversees terrain-response and driveline-mode operation. When this occurs, the module cannot be seen or commanded by other control units, which typically disables terrain-select functions and related all-wheel-drive features. The issue is most often a circuit or connector problem rather than a failed computer.
Open circuits, shorts, moisture intrusion, or corrosion in the All Terrain Control Module harness are the most common causes of lost communication.
A blown fuse, faulty relay, or corroded ground point can prevent the module from powering up and communicating, even though the module itself is intact.
Internal damage such as a defective CAN transceiver or circuit-board failure can cause the module to drop off the network and not respond to other modules.
A short between CAN high and low, or a short to power or ground elsewhere on the network branch, can block communication with this specific module.
This is an uncommon cause; the engine computer is frequently misdiagnosed when the fault actually lies in the module or its wiring circuits.
The vehicle is generally safe to drive, but terrain-management modes and possibly all-wheel-drive functions may be disabled. Avoid slippery or off-road conditions until the system is repaired.
No. Although some drivers are told to replace the PCM, the root cause is almost always a wiring, connector, power-supply, or All Terrain Control Module failure rather than the engine computer.
Resetting the code may temporarily turn off the warning light, but it will return if the underlying circuit or module fault is still present.
Yes. Devices improperly connected to the CAN bus can corrupt network traffic and cause intermittent or permanent loss of communication with the All Terrain Control Module.