When I use to work on those units, they had a fan/limit switch within the unit. This device is a bimetal unit that is located directly within the furnace itself to "sense" the heated air within the furnace. It is used to turn the fan on when the furnace heats up, and off after the furnace cools down. If the fan does not come on, the switch keeps on moving until it reaches what is called the limit switch. At this point the limit switch shuts off the burner to prevent the unit from over heating, and the reset button on the fan/limit switch must be pushed to restart the unit. Additionally, there is a unit located in the exhaust stack called a Stack Switch. This device is hooked to the thermostat. It contains a bimetal heating element located directly in the exhaust stack which also will shut down the burner if the exhaust temperature gets too high, thus the need to push the reset button. Usually when the furnace has to depend on the Stack switch to shut down the burner the Stack Switch is bad. If the fan/limit switch is reaching the limit portion of the switch, then the fan motor is bad.