So a week ago I ran into the problem of my temperature gauge reading zero and - - - F when I look at the coolant temperature on the dash board and my a/c was blowing out hot air instead of cold. I did research and ended up replacing the thermostat a week ago, just today I got into my car to goto work and the temp gauge wasn’t reading again but this time my air conditioner was blowing cool air. Help! Could it be the sensor ? I have a Chevy hhr 2006

Welcome to the site, I’m thinking code P0128, used the wrong thermostat.
it just has to be an AC Delco 131-158 180 degree thermostat and 50/50 Dexcool and distilled water.

I am having the same issue but I am able to reset the gauge by disconnecting my battery and reconnecting... However, 15 minutes into driving it will stop working again. Also, even with resetting, the temperature read on the dash never gets a read and stays showing ——F. Is this also a sensor issue?

Welcome to the site, Mika! Do you have that code? Have you replaced the thermostat?
Changed the Dexcool?

I got a p0010 camshaft position actuator circuit code and p0128 thermostat below regulating temp code.

I changed the intake and exhaust VVT solenoids. But still need to drive it to see if that did anything to solve my problem. I’ll let you know.

X2, get an AC Delco 131-158 thermostat and fresh Dexcool 50/50 coolant.