Should change the thermostat, also......especially if the code was P0128....

My son replaced the radiator and reservoir and thermostat. Behind the thermostat it looks clean even before he flushed it. Put in new coolant and it'* been running good all week. We still haven't located the temp sensor.

Temp sensor is just below the thermostat housing.... black and yellow wiring I think....

Ok thanks

I had the same problem in my Malibu and learned I had the wrong thermostat in it. I put an AC Delco from the dealer with the right temperature and the code went away and has been running fine for 12 months