The heat exchanger is the metal chamber at the core of a gas furnace. Burner flames heat its walls, your blower pushes household air across the outside, and the toxic combustion gases inside — including carbon monoxide — are kept physically separate and vented out the flue. When it works, you get heat and the exhaust never touches the air you breathe.
Decades of heating and cooling make the metal expand and contract until it fatigues and cracks, or rust eats through it. A cracked heat exchanger lets combustion gas leak into the airstream, so it is a carbon-monoxide hazard, not a performance complaint. Warning signs include a CO-alarm trip, soot, a flickering yellow or orange burner flame instead of crisp blue, and a sharp odor when the furnace runs.
A cracked or rusted exchanger cannot be safely welded or patched — it is replaced. The part itself is $350 to $850, but labor dominates: a full replacement typically runs $1,250 to $3,000, around $1,750. Two things make this the classic repair-versus-replace decision. First, many warranties cover the part but not the labor, so an 'in-warranty' job can still cost over a thousand dollars. Second, on a furnace 10 to 15 years old the labor alone approaches the price of a new unit — the rule of thumb is to replace when the repair reaches roughly half the cost of replacement. An honest shop confirms the crack (a visual inspection or combustion analysis), shows it to you, and quotes both paths.