A: I know of one insurance company that says the motor and belt is the most likely parts to cause a fire within your furnace. This company will treat even an old furnace as new if the motor and belt are new. I am sure statistics will bear the fact that most fires are started with the motor and belt. Other parts that may wear out are the thermal couple and the glow plug igniter. Some other parts I always check include the filter and chimney. Finally if your older furnace has a humidifier it is most likely no longer working. I am not a big fan of humidifying your home in this way and well over half of the furnaces I check in this category have humidifier problems.