I understand that a faulty engine thermostat, as well as the engine coolant temp sensor can set this code. Since I'm getting intermittent temp displays, I believe the ECT sensor has gone bad. I would appreciate any comments on this issue, and would also like to know how difficult it is to change.

Thanks

 

Most common is a failed thermostat. If the engine does not reach operating temp.with in a given time the computer sets a low temp code. It will also shut off the coolant temp gauge and your digital read out will show --- where the temp reading would be.

 

You can clear the codes after the engine warms up and all your gauges will come back until the next cold start up. This will confirm it's just a thermostat and not the sending unit.

 

Use the SEARCH box. There is lot's of info on P0128 and step by step help.

 

Thanks to all who have responded; just ordered an AC-Delco replacement thermostat. Hope to change it out this weekend. What brand of anti-freeze do you recommend?

 

There are a lot of opinions on antifreeze type. My opinion is to put in what the car was designed to have.
if the car came with dexcool then that's what I put back.
If you do change the type of antifreeze just make sure you flush the system well. Some types of coolant don't play well with others.

 

Dex-cool it will be.

 

if you buy the Dex-cool concentrate ensure to mix it well 50-50 with drinkable water a 1 gallon jug will make 2 gallons mixture enough for your coolant drain and fill. and here is a fine thread for the how to part, https://www.chevyhhr.net/forums/how-tutorial-library-21/how-save-%24350-00-thermostat-replacement-2-4l-p0128-24493/

 

You do see the SEARCH box, top center of every page?