So while driving down the freeway in my 2008 HHR LT with the 2.4L engine yesterday the check engine light came on and the temp gauge quit working. When I got home I scanned for a code and got the P0128 (Thermostat) code. Funny thing is I replaced the thermostat just a few months ago when the car hit 90K miles with one of those expensive ones that if it fails, it will fail open instead of closed. So now I have to decide whether I should replace the Engine Coolant Sensor this time or the thermostat all over again. This is like Russian Roulette!

 

Here's something to read:
https://www.obd-codes.com/p0128

 

You need to use the AC Delco thermostat... Nothing else.

 

Even those expensive ones?
Was it in fact an AC Delco 131-158

https://www.partsgeek.com/ss/

 

Stuck open is the cause of P0128.

It is a really bad idea to publish your email as your screen name on the internet, you can change it by clicking on "contact us" at the very bottom of the page.

 

I replaced my thermostat yesterday for the same reason. OBD P0128.......cleared the code and everything works correctly.

I paid close to $70 for mine thru a local Chevy dealership. Could have waited and got a lower (half the) price online, but I need the HVAC defrost feature to operate at peak efficiency as the weather where I live is currently wet, cold and rainy.

 

Thanks for the link, good information!

 

After clearing the code I drove it yesterday and I noticed the coolant temperature came up quickly and the check engine light did not activate. Maybe it was just a fluke before, I will closely monitor it and I it does it again I'll pick up the ACDelco thermostat. Thanks for your advise!

 

I'll have it changed, thanks!

 

Is it possible for the temp read out to be working, but the thermostat still be bad? My 06 2.2 threw this code, and besides running about 10º cooler then normal, all the readouts seem fine.