Drew25

Hi everyone. I recently experienced the flatlined temperature gauge as well as the digital readout on the dash. In reading all the forums I went ahead and changed the thermostat. That did not solve the problem so I then changed the temperature sending unit. It worked for one day and now I'm having issues with it getting up to proper temperature but I'm also having issues with the fan running when it's not supposed to. And of course my air conditioning is not working either because of all this. Is there another temperature sending unit somewhere? Like on the radiator or cooling fan assembly perhaps? I've only had this for a week and it's driving me nuts! In one form post I read about how to reset the ECM by pulling the 50 amp fuse located on the battery for 10 minutes and then putting it back in and letting it reboot. I've done that as well and I'm still having the problem. Am I perhaps doing something wrong? Thanks for any help you all can give!

Cat Man HHR

What brand thermostat did you put in?
The 50 amp fuse at the battery is power for the Body Control Module (BCM)
When you changed the T-Stat and replaced the coolant did you raise the front of the car so air could be "burped" out of the system?
Venting air out of the HHR is a major problem that's why raising the front up helps.
If you don't have ramps, park it uphill.

donbrew

Exactly what does "the fan running when it's not supposed to" mean? Only one temp sensor. The t-stat's job is to get the temp UP quickly.

Diagnosis: you did not heed all of the posts that say to get ONLY GM 131-158 t-stat.

Drew25

I honestly did not see the post stating to use only the GM part before I changed it out. The brand I used was Fail-Safe. And I did not before last night see the post about raising it up to burp it out. So I will be trying that today. Also the fan running when it's not supposed to means it's running when it's well below 180 degrees. Like last night it showed the temperature is 118 degrees and it was running full force and the air conditioning was not on

Cat Man HHR

When air is in the system the coolant sensor will read that temperature because of no coolant around it.
Redo what you have done with correct T-Stat (Delco) raise the car and get the air out of the system.
Bet you it might correct your problem.

Drew25

thank you! I will try that. Had a mechanic tell me that if I changed the t-stat and temp sending unit then the cluster gauge is bad and needs to be replaced. Now to find the t-stat. Amazon has it but cant get it to me until Tuesday. Does Napa or O'Reilly stock it? Autozone doesn't

Drew25

Also Ac is blowing warm air now as if I'm trying to use the heater when I'm not

Oldblue

The wrong tstat will cause the A/C to shut down.
Put in the correct AC Delco 131-158 thermostat.
Burp the Dexcool.

donbrew

The mechanic doesn't know anything about the car.

The only reason the fan should be on is if the temp gets up to 217F then it will turn off after it get down to 180F or if the A/C compressor is running.

The A/C compressor will be disabled in the presence of a code P0128 (didn't get to 180F fast enough) which also flatlines the temp readings. This is usually intermittent depending on ambient temperature.

The A/C compressor will run if the mode is set to defrost or recirc, with no indicator.

Maybe some wires are crossed or shorting.

Drew25

ok so I replaced the thermostat with the AC Delco one and burped the air out. Fan still comes on well before 180 degrees and without me trying to run the AC or something. Do I need to disconnect the battery to reset the system since I also changed out the temp sending unit? Never been so frustrated by a car before