Oh man, I just dealt with that problem in my car. Does the warning turn off maybe 30 seconds after you start your car/ turn off your car? And when you push the check control, it says everything is fine?
At first (this is after topping off the coolant and that not working) I replaced the all the ties at the end of the hoses, thinking it might be a leak. The warning light kept coming on so then I checked all the hoses and found no tears. I finally just said "F-this" and replaced the entire radiator and sensor (gee, that was fun...NOT) and problem fixed.
Basically, I'd just check the sensor to make sure it's not faulty.