Q: engine overcooling; engine fan always on!

Hi Alex....yes, that sensor could be a possibility but there are other components in the circuit as well such as relays (that can stick on) and, although not as likely, there are possible wiring faults that can cause continuous power to be supplied. The only way to condemn the coolant temperature sensor is to test it per the procedure in the Factory Service Manual. If you called a professional mechanic, he or she would test it before making a decision. If a sensor is replaced, there are other issues to be wary of such as making sure that when the cooling system is refilled air is not trapped in the system. Trapped air can cause overheating. Unless you can test the sensor yourself, you will have the least aggravation if you let a professional mechanic diagnose this, repair it, and make sure that no new problems are introduced during the repair. If you desire the mobile service that YourMechanic offers, please simply request cooling fan malfunction diagnostic and the responding certified mechanic will take of this promptly for you.