Check engine light- code P0128

07 Pontiac Grand Prix base model 3.8L. Replaced engine and thermostat in April. Check engine light comes on and I went to get it read and it is saying thermostat or gasket. I was not experiencing any issues overheating, etc. a few days later the light comes off, do I still go get this looked at or leave it for now? I do have a warranty on the part still.

That code usually means the thermostat is stuck open.