Probably a failed temperature sensor. One behind the throttle body/water outlet, one in the radiator.

 

As Blackburn stated probably just a faulty sensor or even check your water pump

 

Probably the temp sensors.

 

To test if it is the temp sensor you can disconnect it and put a resistor in it the connector, something like 200 ohm. This will simulate it working and if it goes back to normal there you go. There is two temp sensors, one at the water outlet on the drivers side of the engine and one on the passenger side of the radiator.

 

I dont think anyone is gunna have a 200 ohm resistor in their cabinet lol

 

That was a random number. Anything around there would work. If I remember right I used a 140 ohm.

 

UPDATE……Have replaced both sensors and the thermostat housing and STILL having the same issue 😩 It’s not giving a code (never has) after hubby disconnecting the battery it still randomly turns the fan on (super loud) and no temp gauge again……WHY???? Please help (and it’s not the water pump)

 

Make sure you have completely purged the system of air.

 

Start at the beginning. Why was the thermostat replaced? What issues did you have before the replacement of the thermostat? What engine you have?

 

Hi did you ever find the fix for this issue my 2016 camaro rs has been having the same exact issue and whenever I unplug and replug the negative terminal on the battery it fixes the temp gauge for a few days and then it’ll happen again randomly when driving