2001 Buick Regal - P0128 code

I had my Service Engine light come on a few days ago. I drove it very short distances to and from work during the week, but then the light turned off a couple days ago. I finally got the chance to check it today and was given the P0128 code stating the thermostat coolant temp was below what it is supposed to be. Since the light went off a couple days ago, should I go ahead and replace the thermostat? The coolant level is fine, and it drives fine.

Thank you in advance for any help.

yeah could be the thermostat unless the coolant is full of air or low