U0165 Moderate Generic / SAE

U0165: Lost Communication With HVAC Control Module — Rear

Network and Communication Lost Communication

What to do

Before anything else, check the back of the car for water. Lift the rear carpets and feel underneath, check the third-row footwells and the boot floor, and look up at the headlining for staining if the rear unit is in the roof.

The reason is that this is usually not a coincidence. The rear air conditioning makes condensation just like the front, and that water drains out through a long pipe running through the vehicle. When the pipe blocks — and they do, with dust and debris — the water does not stop being made. It overflows inside the car, and it lands on the electronics. So a damp rear carpet is very often the cause of this fault rather than a separate problem.

Say so if you find any. Clearing the drain is the actual repair, and a module replaced without clearing it will be replaced again.

Two free checks before that. Is there a switch at the front that locks out the rear controls? Most cars with rear climate have one, and a system somebody switched off is not a fault. And does the front air conditioning still work? If it does not, that is probably the real problem — the rear system is normally shut down when the front one is unavailable.

The car is entirely safe to drive.

What this code means

The rear climate control module has stopped being heard from. The general reasoning is at U0100, U0164 covers the front system, and U0466 explains the rear condensate drain — which is the thing to check here rather than a paragraph to read past.

The short version, because it is the commonest real-world cause and it is not obvious. A rear evaporator makes condensate, and that water has to leave the vehicle through a drain running a long way through the structure. Blocked with dust and debris, it does not stop making water — it overflows inside the vehicle, into the rear footwells, the boot floor, or from a roof unit onto the headlining.

And that is very often why the module went silent in the first place. A rear climate code alongside a damp rear carpet, a wet third-row footwell, a musty smell or a stained headlining is one fault, not two — and the module is the casualty rather than the cause. Finding the water is the diagnosis; clearing the drain is the repair. A module replaced without clearing the drain gets replaced again.

Two things to eliminate before any of that, both free.

Check whether the rear controls have been locked out from the front. Most vehicles with rear climate have that switch, and a system somebody deliberately disabled is not a fault.

And check whether the front system works. A rear unit is normally shut down when the front is unavailable, so a rear fault alongside a front one is downstream and the work belongs at U0164.

Where else it fails: the harness runs the length of the vehicle under the sills or the floor, through the areas that take seat bolts, carpet fixings and load-space fittings.

Likely causes

Start here

Less common

Faulty climate control module · specialist
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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