Most likely a thermostat stuck open. If the thermostat is stuck open, the engine temperature won't rise as quickly as expected and the ECM will go into protect mode because the lack of indicated normal temperature rise could also be from a bad sending unit. The humming you hear is likely the engine cooling fan which will continue to run for a few minutes when the ECM indicates a possible overheat condition.

It isn't likely that your engine is actually overheating but don't chance it and get this fixed right away. And do check the coolant level in the recovery/degas tank to make sure it is at the right level when cold. If you see the coolant vigorously boiling in the recovery/surge section of the tank then you do have a true overheat condition. If the coolant level is well below its normal range, be sure to bring it to the proper level before further engine operation (checking and filling is detailed in the manual, NEVER open the pressure cap when the engine is hot because hot coolant will flash boil when pressure is released reducing the boiling point of the coolant).

I would start by replacing the thermostat and it is also probably due for coolant replacement. Be sure and go with a new AC Delco thermostat like the original and NOT from Amazon where the chances of a fake/counterfeit are fairly high. If this doesn't fix the issue, then the next step is to check the temperature sender and its connector. But the symptoms you describe are exactly what many GM owners experience when they replace the factory thermostat with a cooler temperature range thermostat without also reprogramming the ECM.

Rodger

 

Start with the coolant temperature sensor.

 

Hey guys I have a 2015 ATS 2.5L and I got the engine hot AC off message on the display. I also noticed the temperature gauge is stuck and not working (stays at cold). The car runs ok but I am hearing a humming when turning the engine off.

Is this a thermostat or coolant issue?

Sam

I had this happen to me in my 2013 ATS. I took it into shop and it actually is the temp sensor on the back of the head. Good luck!!

 

Hey guys I have a 2015 ATS 2.5L and I got the engine hot AC off message on the display. I also noticed the temperature gauge is stuck and not working (stays at cold). The car runs ok but I am hearing a humming when turning the engine off.

Is this a thermostat or coolant issue?

Sam

Did you check the coolant levels?

 

I took it into the shop they said the coolant levels were OK. Now I am getting the message “engine hot turn AC off” and the temperature gauge does not move at all from cold. The check engine light is now on and there is a loud fan/vent noise that continues for a few minutes even after the car is turned off. Any ideas?

 

Did you read the replies above? There are a some good ideas already posted. From your post it looks like you took the car to a shop and they checked that the coolant levels are ok and nothing else? Did they have any ideas or check anything else? If I took my car to a shop with the symptoms you have and they told me the coolant level is ok and I should go I would go find another shop. Seems like a waste of a trip.

 

Yes thanks for the replies guys. I think it must be either the thermostat or coolant temp sensor like everyone said. I will take it to another shop because I want to diagnose the problem before fixing it (don’t want to waste money on a part that’s working fine) I will keep everyone updated

 

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