My car is overheating

What is your question?

Any suggestions? My car is overheating. There's plenty of coolant with no leaks. I bled the system so the water pump is working but my fan only seems to come on when the AC is working. So the fan is working, but is there some sensor somewhere that tells the fan to come on when the engine gets hot? If so, where and what is it? Or can it be something else? Can the thermostat have something to do with this?