03 Sebring Radiator Fan won't stop

Hello! The radiator fan on my '03 Sebring seems to have become possessed. It won't turn off, though the car is NOT overheated. I've had the car for four years, and this is the first time it's ever done this. People at the local auto parts store have no advice. How about you guys? Do you know if there is a sensor or relay I should check, and where it might be? Thank you very much!
Jon