How do I fix this?, it ran down my bat. Had to pull fuse.
Of course the fan is designed to stay on after you turn off the car to cool it down. And the hotter the engine the longer it will stay on. And if you have a weak battery to begin with, a longer run on the fan can run it down to not starting. If we are past that discussion, then the heat sensor and/or related on-off switch are not working properly.
The radiator temperature sensor may be faulty, or the fan relay might be stuck.