My g/f has a '03 Buick Regal LS and it is the N/A 3.8L (L36?). She briefly threw a DTC for a few days. It is P0128 it was something about coolant temp below threshold or something to that effect... This leads me to think her T-Stat was sticking or she may have a faulty coolant temp sensor as obvious things to check. What would you think, any other suggestions?

The MIL light would come on every once and a great while. It is now off and has not been on in a few days. I ran codes and it was still in the memory, if anyone has any suggestions or if this is a common thing on these things let me know. Thanks for your time.