I still have a flatlined coolant temp after changing both the thermostat and the temp sensor. I havent read code and its still there. Do i need to clear code before temp sensor works again? I thought if I went ahead and changed both out it would be sure to fix the issue with no temp reading and no AC

08 HHR LS 2.2

 

I actually drove it around and it started working. hoping for good

 

Welcome to the site. Burp the coolant, you should be fine!
You did install an AC Delco 131-158 thermostat, didn’t you?


https://www.chevyhhr.net/forums/how-...-system-60614/

 

the test does not runn until the temp rises. Ergo, the have to drive for a few minutes.
The test is a timeline of how long it takes to get to 158F.

 

Don, how quickly should it reach 158F?

 

I don't know, there is an AlGoreithm. Mine always does within 5 minutes.