I’m going nuts with this car lately.
about 3yrs ago I had to change the o-rings on the coolant crossover pipe as it was leaking, since the tstat and sensor was original (surprisingly still worked and I should have left well alone) I changed the tstat and sensor at the same time as I drained and refilled with new dexcool. Later that same year during the summer car coolant flat lines, so I replaced the tstat and all was good. About 6 months later it did it again, I replaced tstat.
fast forward to today and I have replaced that tstat 5 times and sensor twice and it randomly flat lines and leaves me without AC. And yes i have been using the genuine GM parts as suggested on previous posts on this website.
And yes the coolant level is fine and it has been burped multiple times.
WTH is going on here??? What am I missing?

 

If you’ve installed the AC Delco 131-158 180 degrees F thermostat and it still flat lines, replace the coolant temperature sensor. Be sure to burp the coolant.

 

Are you absolutely sure it is a 131-158? If you go to a GM parts department and ask for "a t-stat for HHR" that is not what they will hand you. Many vendors think "like" "similar" "replaces" "fits" means they are the same item.
The 131-158 does not have the inner gasket that pops off and holds the valve open that others have.

 

I ordered the thermostat 131-158 and sensor 213-4333, both genuine GM parts from Rock Auto. The thermostat is no longer listed on their website.

 

What brand? Not all names are created equal.

 

Those are AC Delco numbers Greg
of the thermostat is not in stock , are you saying you ordered the thermostat before?

 

If the t-stat cost less than $25 it is not the right one.
Was the t-stat physically open when you pulled it out? P0128 means the t-stat is stuck open or the sensor is malfunctioning. Usually the inner rubber gasket gets displaced an holds the valve open; you can see it with your bare eyeballs. A t-stat is closed at any temperature below 180F, like room temp. I should start opening at 180F and stay open until it cools back to 180F.

The information on the RockAuto site is supplied by the vendor not RA. We have hears of a few people that bought "Genuine GM 131-158" (from various vendors) only to find out they were "like".
Right now RA does not have the 131-158 listed, but they have a few that have "OE" in the descriptions.
GM does make several t-stats that will fit and are set at 180F, they fail.

 

I have bought the same tstat and sensor from Rock Auto multiple times. They are genuine parts also came in packaging with GM logo on it. I have the email “paper trail” that shows the exact items bought.

 

well just an update, i replaced the temp sensor again and its been a week, and so far so good. Fingers crossed

 

Awesome.