Check the radiator level first, if its properly filled, then the cooling system my need to be bled out. Theres a bleeder on top the T-stat housing. With the motor running and warmed up, crack it open, if air comes out then you neeed to let it all out. You may need to open it and close it a few times. If straight coolant comes out, then your good. If the light STILL appears, its your level sensor on the side of your radiator on the passenger side. Pull it out, I bet ya a dollar that little tip in there is dirty with dex****. Clean it out with carb cleaner and reinstall it, refill the radiator back to the top and bleed again. If that don't fix it, replace that sensor with a new one and you should be good to go (refill + bleed).
I worked on 3 GPs that had this same issues, all of them had the same problem. All were full and bleed out properly. All of them, had gunk in there sensor and need to be cleaned out like I described. After a good cleaning, they working greating without any problems ever again. BTW, flushing the system won't clean this sensor off in my experiance. Taking it out manually and cleaning it is the only good way. Every year when I flush/refill my cooling systems in my cars, I take out that sensor and clean it manually everytime. Works for me....
