On Wednesday I noticed my car's temp gauge reading at bottom while driving down the highway. The air conditioner was also non-functional. Each of these components were working fine the day before. The car continued operation, and I was able to drive it to a mechanic later that day.

My mechanic was unable to diagnose the problem, but also mentioned the thermostat couldn't close, causing too much coolant to flow and the engine to run much cooler than it should.

As I drove away from the mechanic the temp gauge showed the engine heating up short of where it normally is, and then malfunctioned during the drive. There was also a check engine light, but that went away on its own too.

So I guess... what could cause the temperature gauge, AC, and thermostat to fail almost at the same time?

Edit: Car is a 2006 Pontiac G6 GTP

  • Did he check code history in the PCM? That might point you in a direction.
    – Ben
    Commented Jul 22, 2016 at 18:31
  • Also what engine do you have and is the a/c automatic?
    – Ben
    Commented Jul 22, 2016 at 20:33
  • The dealership cleared the history codes. The engine is a 3.6L and the AC is automatic. I'm out of town the rest of this week, so I've made an appointment for next week to have the thermostat replaced. I'll select an answer then.
    – Raven
    Commented Jul 25, 2016 at 14:26
  • I lied engine is 3.9L
    – Raven
    Commented Jul 25, 2016 at 14:54
  • @DeepDeadPool On modern vehicles the engine computer controls Air Conditioning (A/C) engagement. If the vehicle is in an overheat condition, the computer SHUTS off the A/C system to protect the engine. If your sensor goes faulty and the computer thinks you are in an overheated condition, what you are describing makes sense.
    – zipzit
    Commented Aug 1, 2017 at 6:29

2 Answers 2


At the very least, the thermostat should be changed since it really isn't an expensive or intensive repair. Was this done while you were at the mechanic? This would certainly solve the temp gauge and engine temp.

I don't believe (not 100% sure) this issue would affect the operation of the AC (it would with the heat).

  • It was not done while I was at the mechanic shop. I told him only to diagnose it. Could I ask you to elaborate on "it would with the heat"?
    – Raven
    Commented Jul 22, 2016 at 18:28
  • If the radiator is running constantly, the coolant will be flowing continuously through the heater core. I would imagine this would/could negatively affect the temperature of the air coming out of the heater (perhaps not to a noticeable extent). Since the AC uses an entirely separate system to generate cold air, I don't believe it would impact the temperature of the air.
    – Andrew L
    Commented Jul 22, 2016 at 18:33

The most likely scenario is that you only have a single malfunction: your thermostat. If it's open all the time, the engine will take much longer to get up to temperature (as you've noticed), and some/most AC systems are completely paranoid when the engine isn't at it's operating temperature or isn't cooling normally.

Get your thermostat replaced.

  • Replacing the thermostat fixed a few things. The engine runs at the correct temperature now, and the temperature gauge I see on my dashboard no longer fails. However the A/C is still malfunctioning, or at least doesn't seem to be 100% up to par. The software engineer in me wants to disconnect the battery so I can really turn it off and back on again.
    – Raven
    Commented Aug 4, 2016 at 14:34
  • Disconnecting the battery would reset the computer, and that is a common ting to do. Have you ever had the A/C system recharged (with freon)? After 10 years, it may have sprung a small leak. You can get the pressure of the A/C system tested, if it's a bit low they can top it up.
    – tlhIngan
    Commented Aug 4, 2016 at 15:59
  • No I have not. I'll try that next. Thanks for the suggestion
    – Raven
    Commented Aug 4, 2016 at 17:47
  • Update: too much Freon. No idea how that happened. Must have been like that for years. Well everything works now
    – Raven
    Commented Aug 11, 2016 at 16:52

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