Ken writes by email: “I hope you can give me a hint as to what is wrong with my 2007 Buick LeSabre. Anytime I try to operate the heater, the air-conditioning compressor runs no matter which settings I use on the climate control system. A small amount of slightly warm air comes out of one of the under-dash outlets, and very cold air comes from the other. The passenger’s control has no effect whatsoever.
I disconnected the air-conditioning compressor, which results in very little change. Engine temperature is normal. I have heard that an electronic module controls this function. If this is true, I would like to know which one and where it is located.”
Halderman: From your description, I think the blend door motor is no longer working. The door is stuck in one position. The control head could be the problem as well, causing the door not being commanded to move. If it is an auto climate control, hooking up a scan tool should show some codes or data on the HVAC system.
The air conditioning compressor typically runs all the time anyway if it is auto climate. When it is very cold outside, a sensor will keep the compressor from running until the engine gets to a certain temperature. If the air does not come out the right vents, then my guess would be a control head problem.
Because this diagnosis and repair is complex, I suggest you ask a professional service technician to check this for you to help avoid replacing unnecessary parts.
Have an automotive-related question? Write to Jim at firstname.lastname@example.org.