A faulty heat exchanger can be a serious safety concern, as it can lead to CO production. Signs of a faulty heat exchanger include rust or corrosion on the exterior, soot or debris accumulation, or unusual noises during furnace operation. Additionally, if your furnace is producing a strong, sweet or metallic odor, it may indicate a heat exchanger issue. If you suspect a faulty heat exchanger, it’s essential to contact a qualified HVAC technician for inspection and repair.