How much does it cost to replace a furnace flame sensor?

Prices checked October 2026. Wages: BLS, May 2025.

A flame sensor is a thin metal rod that sits in the burner flame, and it is behind a lot of furnaces that light and then quit. Lennox describes its job simply: it checks for a flame each time the furnace fires and shuts the gas off if there isn't one. Over time the rod builds up a coating that weakens its signal. The control board then decides the flame went out and closes the gas valve, even though the burners lit fine. Sometimes cleaning fixes it. Sometimes the rod or its insulator is damaged and a new part is the better fix.

The range above covers a technician's visit to install a new sensor on a gas furnace. It is the online list price of replacement flame sensors from retailer listings plus about an hour and a half of billed time, half an hour hands-on and an hour for travel and diagnosis. The hourly rate starts from HVAC technician wages in your metro or state (Bureau of Labor Statistics, May 2025) and is multiplied to reflect what a service company has to charge to stay in business.

What moves the price

Cleaning versus a new sensor

Many technicians clean a coated sensor first and test the signal. If the reading comes back to normal, the bill may only be the service call. A cracked porcelain insulator, a bent rod or a corroded lead means replacement. The invoice should say which was done.

Part fit

Flame sensors differ in rod length, mounting and connector. Some furnaces take a generic rod, while others need the brand's exact part. Retail prices run from inexpensive universal sensors to brand parts at a few times that.

Finding the real cause

Short cycling has other causes: poor grounding, a weak or cracked burner flame, low gas pressure or a control board problem. A careful technician checks these before reaching for parts, and that time is part of the diagnostic hour.

Regional wages and timing

Labor is billed from local HVAC pay at the 25th percentile, median and 75th percentile, with a higher markup at the top end. Calls on the first cold weekend of the year often carry surcharges that land above the range.

Repair or replace?

Always fix a flame sensor problem. The part is inexpensive, the repair is quick, and a furnace with a bad sensor will keep shutting itself off. It also tells you very little about the rest of the furnace. Bigger decisions only come up if the same visit turns up a cracked heat exchanger, rusted burners, a failing inducer or a control board fault, or if the furnace is past 20 years old with a string of recent repairs. DOE's analysis puts average furnace life at about 21 years.

Run the numbers for your unit.

What a complete quote lists

• What the technician observed, such as a low flame signal reading or short cycling
• Whether the sensor was cleaned, replaced or both, and the part number if replaced
• Checks of grounding, burner flame and gas pressure, or a note that they were normal
• Service call or diagnostic fee
• Labor charge or flat repair price
• Confirmation that the furnace completed a full heating cycle after the repair
• Warranty on the part and the labor

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