U0138 Moderate Generic / SAE

U0138: Lost Communication With All Terrain Control Module

Network and Communication Lost Communication

What to do

Do not take the vehicle off-road or anywhere demanding until this is fixed.

The terrain modes — mud, sand, snow, rock — will not work, and the important part is that the vehicle looks and feels completely normal while being set up entirely for tarmac. Traction control will cut in early rather than letting the wheels spin as they need to in soft ground, the gearbox will change up when it should hold a gear, and hill descent control may not be there when you want it.

A driver who selects a mode, sees nothing happen and carries on is relying on settings the vehicle has not applied — which is exactly the situation to avoid on a slope or in mud.

Two things worth mentioning. Check your carpets, especially around the centre console and under the front seats: this module usually sits near the mode dial, which is the wettest and most walked-on part of the cabin, and a damp carpet is a real clue.

And if other warnings are on too — four-wheel drive, differentials, suspension — those are probably the actual fault. This system does not do anything itself; it tells the others what to do, so when one of them is unhappy terrain mode stops working as a consequence.

What this code means

The all terrain control module has stopped being heard from. The general reasoning is at U0100, and U0439 sets out what terrain mode actually does and why losing it removes more than a selector.

The short version of that argument, because it is the whole safety content: this module does nothing itself. It tells everything else what to do — throttle response, shift points, the transfer case and couplings, the differentials, traction control thresholds, hill descent control, sometimes ride height. When it goes quiet, every one of those reverts to its standard on-road setting.

And the vehicle looks and feels entirely normal while being configured for tarmac.

So: do not take it anywhere demanding until it is fixed. Traction control will cut in early rather than allowing the wheelspin that clears mud or sand; the gearbox will change up where it should hold; hill descent control may be unavailable on exactly the descent it was wanted for. A driver who selects a mode, sees nothing happen and proceeds anyway is relying on configuration the vehicle has not applied.

One thing specific to silence rather than bad data. Because this module is a coordinator, it is often mounted in the cabin near the selector dial — centre console, transmission tunnel or under a seat. That is the wet, spilt-drink, foot-traffic part of the vehicle, and it is also where a footwell leak ends up. A damp carpet is worth mentioning before anything is ordered.

And as at U0439, look past this module first. A fault in the transfer case, the couplings, the differentials or the suspension makes terrain mode unavailable as a consequence, so this code alongside any of those is downstream and the work belongs upstream.

Likely causes

Start here

Less common

Blown fuse or failed supply · easy
Damaged data bus wiring · moderate
Another module holding the bus down · specialist

Symptoms

Components involved

Information, not instruction

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Community experience

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