Furnace ignitor and flame sensor replacement cost

The ignitor lights the gas at the start of each heating cycle. In most modern furnaces it is a hot surface ignitor, a small ceramic element that glows orange. Some furnaces use a spark ignitor instead.

The flame sensor is a thin metal rod that sits in the burner flame. It tells the control board that the gas has lit. If it cannot detect a flame, the board shuts the gas off within seconds as a safety measure.

Neither part is expensive. You are paying for a licensed technician to come out, test the ignition sequence, confirm which part has failed and fit the right replacement. Fixr lists the service call or diagnostic fee at $75 to $200 and technician labor at $75 to $150 per hour.

Diagnosis matters here because several faults look alike. A furnace that clicks and will not light, or lights and goes out, may have a bad ignitor, a dirty flame sensor, a pressure switch problem, a gas supply issue or a failing control board. The page on a furnace that won't ignite walks through how those differ.

• The part. Universal hot surface ignitors fit many furnaces and cost less. Some models need the manufacturer's own part.
• Material. Older silicon carbide ignitors are fragile. Silicon nitride ignitors are tougher and usually cost a little more.
• Access. Parts are easy to reach on most furnaces. A furnace in a tight attic or crawlspace takes longer.
• Timing. Ignition parts usually fail when the furnace is working hardest. A no-heat call at night or on a weekend carries an after-hours charge.
• Flat rate or hourly. A flat-rate company charges one price for the repair. An hourly company charges the fee plus time and the part.

A flame sensor rarely breaks. More often it gets coated with a thin layer of residue from the flame and stops reading correctly. A technician removes it, cleans the rod gently and refits it, and that is commonly done as part of a furnace tune-up.

The classic sign is a furnace that lights, runs for a few seconds and shuts off, then tries again. The page on bad flame sensor symptoms covers the signs and what cleaning involves. If the rod is cracked, bent or badly corroded, or the porcelain insulator is broken, it needs replacing.

Ignitors cannot be cleaned back to health. Once the element cracks or its resistance drifts out of range, it has to be replaced.

An ignitor or flame sensor is a wear part. Replacing one is normal upkeep and no reason to replace a furnace.

Pay attention if ignitors keep failing. Repeated failures can point to short cycling, a voltage problem, a dirty burner or an oversized furnace. Ask the technician to look for the cause. On an old furnace with several other faults, add up the repairs before you approve the next one. This Old House's rule of thumb is that a repair is generally worth it if it costs less than half the price of a new furnace, and ignition parts fall far below that. See the HVAC cost guides for other repair prices.