How do I know if my furnace is dying?

A dying furnace usually shows more than one of these signs at once: the burner flame looks yellow or orange instead of steady blue (possible cracked heat exchanger), the furnace short-cycles on and off without fully warming the house, gas bills climb without a change in thermostat habits, rooms that used to stay even now have hot/cold swings, you hear banging or rumbling on startup, or you see rust, soot, or scale where the flame meets the heat exchanger.

Any of those on their own can be a repair — but three or more on a furnace over 15 years old usually means replacement is cheaper over 2-3 years than a string of fix-it visits. A carbon monoxide alarm going off near the furnace is an immediate shut-off-and-call scenario, not a 'watch and see' symptom.