OBD-II trouble code
U0138: Lost Communication with All Terrain Control Module
Another module on the vehicle network can no longer hear from the all-terrain / terrain-mode control module — the computer behind the drive-mode selector that retunes traction, throttle, and four-wheel-drive behavior for sand, mud, snow or rock. The vehicle still drives, but it drops back to its default on-road calibration.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $0 – $1,100
- DIY difficulty
- Intermediate DIY
What does U0138 mean?
Trucks and SUVs with a terrain-mode selector — the dial or button set labeled with icons for snow, sand, mud, rock crawl and so on — carry a dedicated control module for that feature. Depending on the manufacturer it goes by names like the all-terrain control module, terrain management module, or drive mode control module. Its job is coordination rather than actuation: when you select a mode it tells the engine module to soften or sharpen throttle response, tells the transmission module to change shift scheduling, tells the stability and traction control system how much wheel slip to tolerate, and on capable four-wheel-drive systems tells the transfer case and differential locks what to do. All of that coordination travels over the vehicle's communication network, typically a CAN bus.
U0138 sets when a module that expects to hear from the all-terrain control module stops receiving its messages for longer than a calibrated interval. The code is about silence on the network, not about a mechanical failure in the drivetrain. The module has either lost power or ground, lost its connection to the bus, failed internally, or is being drowned out by a network problem that is not really its fault at all.
The practical consequence is a loss of capability rather than a loss of mobility. With the coordinating module silent, the systems it normally instructs fall back to their default calibrations — which are the ordinary on-road settings. The vehicle drives, steers and brakes normally. What you lose is the ability to select a terrain mode, and on some platforms the associated hill descent control, low-speed crawl assist, and automatic differential locking go with it. That is a low-stakes annoyance on pavement and a genuinely serious one if you are relying on those features off-road or in deep snow, which is why it is worth fixing before a trip rather than after.
As with every lost-communication code, the causes are mundane and electrical. A blown fuse or a corroded ground leaves the module unpowered and therefore silent. Chafed, rubbed-through or water-damaged bus wiring breaks the path. Connectors corrode, especially on vehicles that actually get used in the mud the module was designed for. A weak battery or a failing alternator produces intermittent dropouts that come and go with no obvious pattern. And a single failed module anywhere on the bus can disrupt traffic for every other module, so U0138 stored alongside a scattering of other U-codes usually points at the network itself rather than at the terrain module.
Common causes
- Blown fuse or lost power feed to the all-terrain control module
- Corroded, loose, or high-resistance ground for the module
- Chafed, broken, or water-damaged CAN bus wiring in the module's branch of the harness
- Corroded or backed-out pins at the module connector — common on vehicles used off-road
- Water intrusion into the module or its connector after deep-water driving or a leaking body seal
- Weak battery or failing alternator causing low-voltage dropouts
- Another failed module on the same bus disrupting network traffic for everyone
- Damaged bus terminating resistor or a break in the twisted pair
- Aftermarket accessory wiring spliced into the network or into the module's power feed
- Internally failed all-terrain control module (verify power, ground and wiring first)
- Module replaced but never programmed or configured to the vehicle
Symptoms
- Terrain / drive mode selector does nothing, or reverts immediately to the default mode
- Warning message such as service four-wheel-drive, service drive mode, or terrain management unavailable
- Hill descent control, crawl control, or automatic differential locking unavailable
- Check engine light and other warning lights may be on
- Multiple U-codes stored across several modules if the bus itself is at fault
- Symptom is intermittent and appears mainly over rough roads or after driving through water
- Vehicle otherwise drives, steers and brakes normally
Diagnostic steps
- 1.Scan every module on the vehicle, not just the engine module, and record all stored and pending codes. A single U0138 points at the terrain module's own circuit; U0138 alongside many other lost-communication codes points at the bus.
- 2.Check the fuses feeding the all-terrain control module and confirm battery voltage at its power pin with the key on. An unpowered module is silent, and a blown fuse is the cheapest possible fix.
- 3.Test the module's ground with a voltage-drop measurement rather than a simple continuity check — a ground can pass continuity and still be too resistive to work under load.
- 4.Test the battery and charging system. Low or unstable system voltage produces exactly this kind of intermittent communication loss and is a frequent cause of chasing the wrong part.
- 5.Locate and inspect the module connector for corrosion, green residue, backed-out pins, water staining, and a properly seated locking tab. On off-road vehicles, look specifically for evidence of water intrusion.
- 6.Inspect the harness routing to the module for chafing at body pass-throughs, near suspension components, and anywhere a skid plate or underbody shield could have pinched it.
- 7.Measure resistance across the CAN high and CAN low lines at the diagnostic connector with the key off. Roughly 60 ohms indicates both terminating resistors are present and the bus is intact; about 120 ohms or an open indicates a break or a missing termination.
- 8.Perform a wiggle test along the module's harness branch with a scan tool watching the module's status, working from the connector outward. A dropout while wiggling locates the damage.
- 9.Repair whatever is found, clear codes, and verify by cycling through every terrain mode and confirming the module reports present on a full network scan.
- 10.Only after power, ground, connectors and bus integrity have all been confirmed good should the module itself be replaced — and note that most require programming or configuration to the vehicle afterward.
Repair cost
$0 – $1,100
The low end is zero because a blown fuse, a loose connector, or a corroded ground that only needs cleaning are all real fixes with no parts cost. Diagnostic time to trace a network fault typically runs $100-$200. Wiring and connector repair commonly lands at $150-$500 depending on how much harness has to be opened and whether the damage is somewhere accessible. A replacement all-terrain / terrain management control module with the required programming is the expensive end at $500-$1,100. Because so many of these codes turn out to be power, ground, or connector problems, proper diagnosis is the single best money-saver here.
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.