What furnace repair costs, part by part.

Flame sensor — roughly $80–$250. This is the single most common furnace fault and the cheapest real repair. The sensor is a thin metal rod that confirms a flame is actually present; when it gets coated in residue, the furnace lights and then shuts down after a few seconds as a safety measure. Cleaning or swapping it is quick work.

Igniter — roughly $150–$350. The igniter glows hot to light the gas, and it wears out on a predictable schedule — usually every 4–7 years. A furnace that hums and tries but never lights is very often this. The part itself is inexpensive; most of the cost is the visit.

Capacitor or relay — roughly $150–$400. Small electrical components that start and run the blower. They fail suddenly and without warning, which is why a furnace can be fine one day and dead the next.

Blower motor — roughly $400–$900. If air isn't moving even though the burners light, the blower is the usual suspect. Variable-speed motors sit at the top of that range.

Control board — roughly $400–$900. The furnace's brain. When it fails you often get erratic behaviour, blinking error codes, or a system that ignores the thermostat entirely.

Heat exchanger — roughly $1,000–$2,500, and the decision point. This is the metal chamber separating combustion gases from the air you breathe. A cracked one can leak carbon monoxide, so a technician will red-tag and shut down the furnace rather than leave it running. On a furnace over about 12 years old, this repair usually costs enough that replacement is the better use of the money — especially since a crack often signals the rest of the unit is equally tired.

The rule of thumb that actually works. Multiply the repair quote by the age of the furnace in years. If the result comfortably exceeds the price of a new system, replace it. Under that, repair. A $300 repair on a 9-year-old furnace is easy maths; a $1,800 repair on an 18-year-old one almost never is.