Engine coolant temperature sensor
 

I have had this same problem with my 2008 hhr . I would be driving around and all of a sudden my ac would stop blowing cold air and i would notice the temperature gauge also stopped showing the engine temp. I connected a scanner and got the same code for engine temp below threshold. I did some looking around online and noticed everyone was saying the same thing either the coolant temp sensor or the thermostat was stuck open. So i stopped by my local autozone to pick up a coolant temt sensor and a thermostat. I noticed that autozone offered a coolant temp sensor with the coolant temp sensor plug. So i picked it up with the thermostat. Got home and waited for the car to cool off. Once the car was cool i place a drain pan under the car and replace the thermostat and coolant temp sensor. Then i replaced the plug for the coolant temp sensor. I topped off the cooling system and waited to see if the temp gauge was going to work. All of a sudden the tach is jumping and so is the fuel gauge. I unplugged the coolant temp sensor and plugged in my old sensor and the car ran fine except for the ac and temp gauge. I did so checking and found the sensor autozone sold me was a universal sensor. I talked the the store manager and he said to bring back the sensor for a refund. So i order an ac delco coolant temp sensor from Amazon and it does the same thing as the sensor from AutoZone. :hissyfit: anyone have any ideas?

Welcome.

Posting the codes when the CEL is on is pretty much mandatory if you are looking for advice. We'll have to assume you mean the semi-popular p0128.

Now that you have replaced the thermostat and have put the original temp sensor back in, is the CEL lit? If so, what is the code with and without the new ACDELCO sensor?

Did you try disconnecting the negative post on the battery for a few minutes to reset the codes?

And posting links to the items you installed would be helpful as well - as we can often point out a more suitable part that we pretty well will know is gonna work. (ie: about the only thermostat we can recommend is the ACDELCO one)

Polarity?

https://cimg8.ibsrv.net/gimg/www.che...bba6463a86.png

The code i had was the p0128 . As far as polarity goes i matched the wires to the plug so that i connected the wires with both plugs in the same position and did one wire at a time to make sure of no mix ups. Right now the temp gauge is reading correctly when compared to the coolant temp on a scanner. But now the the ac does not work and the gauges are jumping. The sensor i removed had makings of Delphi 126 , the second sensor was an ac delco sensor 213-4514 i believe was the part number.

Note that it is not exactly polarity that counts. If the 5V ref gets shorted everything goes crazy.
The brand and part number of the t-stat is very important ACDelco 131-158

Yes, P0128 , that thermostat as Donbrew states. Trust us , we know a thing or two about them and you’ll need to burp the cooling system.

I changed the t-stat and check engine light went out. A few days later the temp gauge stopped working and the check engine light came back on. Autozone read the code for a bad t-stat. I am thinking about changing the temp sensor. Any thoughts out there?

You didn't buy the more expensive ACDelco 131-158, did you? Any other thermostat is a potential problem, that you are experiencing.

My fix
 

My problem turned out to be the reciever under the headliner. I guy i met that worked for a gm dealer told me to check the reciever behind the headliner( I think it is 8 inches from the rear view mirror.) I unplugged the reciever and everything worked like it is supposed to. I believe the reciever is for the tpms system. I do not know how they are linked but it solved my problem.

Has nothing to do with coolant temp; has everything to do with TPMS. p0128 can be intermittent because it is based on the time the coolant takes to get to 180F; warm startup gets to 180F faster than cold.