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2008 Pontiac GT, AC only works when coolant shows reading


I have a 2008 Pontiac GT, regularly maintained and all runs fine. Just one problem that has popped up over the last few weeks since the warmer weather has arrived: On the computer readout in the dash, if the “Coolant” display shows a temperature (say, 83C or what have you), then the AC works. However, much of the time the Coolant reading shows “—” (3 blank dashes), and the AC does not work. It’s still been quite cool around here (Toronto area) so I haven’t needed the AC much but it’s heating up and I’d sure love to take care of this if possible. Thanks for any help!

As you have noticed there seems to be a problem with your coolant temperature reading. It’s probably caused by a faulty coolant temperature sensor. This sensor measures the temperature of the coolant and reports it to the car, so it can adjust fuel mixture accordingly and operate the radiator fans as needed, among other things. If your car doesn’t know what the coolant temperature is, or suspects that it is out of normal operating range, it will disable the air conditioning to lessen the load on an engine that may be overheating.

Any local garage should be able to fix this for you.

Thank you asemaster, I’ll try to get to one in the next couple days. Do you have any idea roughly how much this would cost so I can be at least slightly prepared?

Prices vary widely, and I’m a little shaky on what a CDN$ is compared to US$. But I’d suspect $25 for a sensor and an hour to an hour and a half of “labour”, which runs about $100/hr here.

GM used a ceramic tipped ECT sensor in some models and this is prone to cracking. Make sure that it is replaced with a brass tipped sensor. They are usually not too hard to get at or replace, maybe an hour labor and $45-60 USD for the sensor itself.

The AC compressor is turned on and off by the computer. If the computer doesn’t get all the inputs it needs, it won’t turn on the compressor. It needs the coolant temp info because it will shut off the AC if the engine is overheating. That is why the AC stops working.