The heat exchanger is the metal wall inside your furnace that keeps fire and exhaust on one side and the air you breathe on the other. Burners heat the metal, your blower pushes household air across its outside, and exhaust goes up the flue. A crack in that wall breaks the separation, which is why this is the one furnace finding where shutting the system down is the correct call until the situation is fully understood.
We want to be straight about two things at once. First, cracked heat exchangers are not an everyday finding, and most furnaces in La Vergne finish their working lives without one. Second, when a crack is confirmed, it is not a part to watch or patch. The furnace stays off, and the real conversation is repair versus replacement of the furnace itself. Anyone who tells you a crack is certain without showing you evidence is skipping steps, and anyone who shrugs one off is skipping the safety part.
Older furnaces in Heritage Valley and Lake Forest Estates, many now well into their second decade, are where we check most carefully during furnace repair and fall tune ups. Age alone does not crack a heat exchanger, but years of heating and cooling cycles expand and contract the metal thousands of times, and that fatigue is what eventually opens cracks, usually near welds and bends.