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Hot/cold climate control

I have a new problem with my’04 Park Ave Ultra that I’ve had for 4 years (125K miles). The climate control heats up the cabin to a nice 80 degrees for the first 30 minutes of driving, ambient temp 30-40 degrees, but probably in any cold temps. Then it begins to pump in cold air through the defrost at the same time that it is heating through lower ducts. I believe this can be a desired thing, especially when there is sunlight coming in the cabin, but in my car this does not depend on sunlight and it is putting way too much cold air in through the defrost and it does not stop adding this very cold air, so that the cabin soon becomes uncomfortable in spite of the warm air coming in down below.

I took the car to the local repair shop and they reached a conclusion they admitted they weren’t too sure about: they said they thought it might be the control module in the dash, about the size of a radio, sending a signal to add some cool air, but never sending the signal to STOP sending the cold air. I also wasn’t too sure about this diagnosis, but the situation was made worse by the fact that they say they cannot buy that control module, it has to be sent back to Buick for repair, cost unknown. Interestingly, on the part they pulled from my dash they found that yellow inkpen writing like a salvage yard might use, suggesting maybe the part had been replaced before, though not during my ownership.

Then the shop said they found a “Buick LeSabre Climate Control 1623 Remanufactured” unit for sale on Amazon for $129, but even this would need to be programed (“flashed”?) by GM to work on my particular car. Again, not sure what the final cost would be…or whether it would solve the problem.

So I decided to break down and take the car to the dealership for a diagnoses. They said they poured a diagnostic fluid in the coolant and decided the car has a very early stage head gasket leak, leading to air pockets in the cooling jacket, which results in the unwanted cold air from defrost. This does not make sense to me, not when warm air continues to come in through lower ducts. They want $2,000 to fix the head gasket.

My own sense is that this is a duct door or actuator for a door, but I don’t know which door or how to diagnose it.

I’d love to hear thoughts on this. Thanks.