Q: Fan doesn't come on unless defrost is on.

Hi there. I am assuming you are referring to the radiator cooling fan. If it works when the AC or defrost is on and not when the engine starts to warm up, then that can be a circuit issue or a temperature sensor problem. If the engine seems as if it is getting hot and the fan is not turning on, and the gauge is still showing cool, then the sensor is more than likely faulty.

I start by checking the sensor reading, and if it is fine, then I test the circuit. It can also be a relay problem, so the entire circuit will need to be tested. If you are planning on having the fan issue repaired, consider having a certified mechanic, such as one from YourMechanic, come to your home or office to determine if you need a sensor replacement.