The furnace problems that are about safety first and comfort second.
A heat exchanger separates the burning gas and combustion byproducts from the air that gets blown through your ducts and into the house. A crack in it can let those byproducts, potentially including carbon monoxide, mix into that airstream. It's not something to diagnose by eye through an access panel — a proper inspection uses combustion analysis and other checks a licensed contractor is equipped to run, and it's not a job to leave half-checked because the furnace 'seems fine otherwise.'
Airflow problems feed into this in a less dramatic but very common way: a dirty filter restricts the air moving across the heat exchanger, the exchanger runs hotter than designed, and the high-limit switch trips to shut the burners down before it overheats further — which shows up to you as short cycling or a furnace that won't stay running. It's usually a filter, not a failing furnace, and it's the single easiest thing to rule out before anything else gets inspected. Thermostat faults — a dead battery, bad wiring, a miscalibrated sensor — get ruled out the same way, before anyone assumes the furnace itself is at fault.
Describe what the furnace is doing and we'll match you with a licensed contractor who can act on it.
Get a Furnace Contractor OutA short description is enough — we'll follow up to confirm the details and get someone scheduled.