The tires on your car wear from constant grinding and flexing over pavement. It's easy to know when they're dangerous and need replacement because they wear from the outside in and the wear is easily visible.
Your furnace heat exchanger also wears and can become dangerous. It wears from the expansion and contraction caused by the burners as they continually go on and off, heating the metal and then allowing it to cool. The resulting damage can be compared to bending a paper clip back and forth until it breaks. But unlike your car tires, you have no way of knowing how dangerous your furnace is becoming because it wears from the inside out.
Here's a closer look: The heat exchanger in your furnace consists of a series of closed metal chambers that are heated with gas burners. The burning process within the chambers generates deadly carbon monoxide gas, which is safely vented out through the flue on your furnace, just like the tailpipe vents carbon monoxide from your car.
To heat your home, air is blown over the heated chambers. A crack or hole in the chambers allows the deadly carbon monoxide gas out of the chamber and into the air being blown into your home.
Furnaces should be inspected every year for signs of wear or damage and carbon monoxide leaks. It is recommended that you replace your heat exchanger or furnace immediately if damage is spotted.
Operating an older, untested furnace is as dangerous and life-threatening as taking your family out for a drive on a set of bald tires.