is your a/c on all the time?
the sensors that control them are the coolant temp sensor and the a/c press sensor. however, the most likely culprit to your problem might be the relay. but try unplugging each sensor one by one and see if the fans turn off, if not then one of the aux fan relays is sticking and needs to be changed(unplugging the coolant temp sensor will most likely activate CEL, so make sure you can reset it).

nenad