Furnace won't start fan
 

My gas furnace stopped working properly. If left to its own devices, it will light the burner and overheat--tripping the safety switch. It won't start the blower with the thermostat on 'AUTO'. But if I turn the fan control to 'ON' it will cycle properly and heat the home. I just have to leave the fan on all the time.
I'm hoping this is a simple problem, but I don't want to try to start something that might wind up being major.
Thanks,
EW

Well its not real simple, If your furnace has a electronic board you need to look at that board and find a post with a wire marked heat. fire the furnace and check this heat post for a while with the furnace running and see if you get 120 volts on it. If you do then the board is good , if you don't then the board is bad.
If you do get 120 volts then check the wiring going from that post to blower. If wire looks good then you might have a bad winding in the blower motor. Paul