temp sensor problems.
 

So the past few days I have been having problems with the temperatures not reading when I start the car. After driving it for about 5 miles the temp would kick on and start reading with no signs of a check engine light. During the time it didn't read I would not get cold air from the AC also. Then a couple of nights ago I get in it. It stopped working then the check engine light comes on And stays like this for a few days. I have read on here about the thermostat being the problems but the timing of mine failing to turn on is whats throwing me for a loop because it only did it the first time I started it and then stopped reading it altogether. The code when I read it was for temp level. Then today as I go to purchase a coolant sensor for it. it all of a sudden starts reading again. While im at work (oreilly auto parts) I clear the code and now its all fine. Im pretty sure it will again do the same thing. But the question is. Is it the Thermostat or the coolant temp sensor. I ended up buying the thermostat but have yet to put it in.

Mine was doing the same thing a month ago, I replaced the thermostat and after that I have not had a problem.

What was the code when you checked it? P0128 will be the t-stat, "Engine Coolant Temperature [ETC] Below Thermostat Regulating Temperature".

If the coolant temp does not reach a set temp in a pre-set time it will throw the code, the DIC temp reading will be "----", the dial guage will not work and your AC will not work either.

Replacing the t-stat is a pretty easy job, 2 bolts and no gasket as they use a o-ring and are pre-attached to the t-stat.

It's the thermostat - see this link for the replacement procedure:
https://www.chevyhhr.net/forums/how-tutorial-library-21/how-save-%24350-00-thermostat-replacement-2-4l-p0128-24493/