Couple things:
First, whenever you have a light on and get the code read, make sure you have the number of the code. Having the code read and the parts counter guy tell you what is pointless. Most of them are not mechanics and don't know much about your car. Their job is to sell parts, not fix cars. So make sure you get the actual code number so you can do your own research or talk to actual mechanics.
Second, check your coolant level. Not in the overflow tank, but rather remove the radiator cap and check the coolant level in the radiator. If it is low, fill it to the top.
Third, probably wouldn't hurt to do a flush of your cooling system and replace the t-stat. But once we know the actual code, I can tell you better what is wrong.
