Temp gauge drops to 0 and AC stops working intermittently

Either bad Engine coolant sensor, bad thermostat or both. Older engines they are behind the alternator right next to each other so replace them both newer engines they moved the sensor to the other side of the engine back by the firewall. So try one and then the other.

When temp light goes on .i stop car ..wait a minute .but it wont turn over

My son is having the exact same issue. Really don't think its the thermostat as it drops suddenly, and you're not experiencing engine overheat. I imagine your heater works just fine and when you are getting a reading on your temp gauge, it's right in the middle where it should be once your engine warms up. The fact that disconnecting your battery resets the ECM tells me its probably an issue in the ECM module.

Christopher! did your son ever fix the issue?

I have had this problem ever since I bought the 2009 Envoy 3 yrs ago. I changed the engine temp sensor and that seemed to fix it but they (eng temp ga and A/C) went out again. I considered taking drastic action - replace sensor, control panel in car, giving it to car AC repair place, etc. Then I remembered that when the sensor was replaced they had to disconnect the battery to do so and another time when I was looking for the sensor i had to disconnect the battery to look at connections to sensor and when I reconnected the bat everything worked again, So when the AC and temp gauge went out a fedw days ago, I disconnected the neg terminal of the battery, waited an hour and then reconnected the bat and everything worked. Something in the electronics of the car must get out of sync and causes the gauge and ac system to shut down. Disconnecting the battery causes the electronics to reboot which cures the problem!

I know this is an old thread but with so many people experiencing this problem someone may find this helpful. I have an 09 Trailblazer LT 4wd, 4.2l engine. I was having the same issue. Randomly the temp gauge would go to zero, the ac compressor would shut off and it would set a P0128 code (coolant temp below normal operating temp). I replace the coolant temp sensor and no change. When I checked live data (after clearing the code) it would run at about 180 degrees. In my day that was the perfect running temp so I never thought twice about it. So I replaced the thermostat and housing (Napa $28.00). Now it runs at 202 degrees and I havent had a problem since. Why the temp gauge would go to zero I have no idea. For some reason the computer turns off the gauge along with the compressor when the code is set. Why they would do that at a time when you need to know what the coolant temp is has not made any sense to me. Also, when you replace the thermostat don't remove the belt, the alternator and all that. Just remove the battery and battery tray and have a 24" extension with a 10mm wobbly socket, Its a 45 min job. Piece of cake. Good luck.