How to Clean a Furnace Flame Sensor

A furnace that ignites, runs for a few seconds, and then shuts off is one of the most common heating problems in Ontario homes, especially on the first cold days of fall. In most gas furnaces the cause is a dirty flame sensor. The sensor is a thin metal rod that sits in the burner flame and confirms to the control board that the gas is actually burning. When a layer of residue builds up on the rod, the board can no longer detect the flame and shuts the gas off as a safety measure. Most furnaces retry a few times and then lock out, so the house gets colder even though the furnace keeps trying.

You hear the igniter and see the burners light, but they go out within about 3 to 10 seconds. The furnace repeats this several times and then stops. On many furnaces the diagnostic light on the control board, visible through a small window in the front panel, flashes a code; compare it with the code chart on the inside of the furnace door, where a 'flame sense' or 'flame not detected' code points to this problem.

A 1/4-inch nut driver or screwdriver, a flashlight, a fine abrasive pad (such as a fine Scotch-Brite pad) or very fine emery cloth, and a clean paper towel. Do not use coarse sandpaper, which scratches the rod and makes it foul faster. If the sensor needs replacing, a new one usually costs CAD $15–$40 for your furnace's make and model.

1. Set the thermostat to OFF.
2. Turn off power at the furnace service switch (it looks like a light switch on or near the furnace).
3. Close the manual gas valve on the pipe leading into the furnace. The valve is off when the handle is at a right angle to the pipe.
4. Wait at least 15 minutes for the burners and surrounding metal to cool.
5. Remove the front access panel or panels.
6. Find the flame sensor: a thin, usually bent metal rod with a white porcelain base, held by a single screw, sitting in front of the burners with one wire attached. It is often on the opposite side from the igniter.
7. Note how it is positioned, remove the screw, and slide the sensor out. Disconnect the wire only if you need to, and note where it goes.
8. Gently rub the metal rod with the fine abrasive pad until it is clean and shiny. Avoid the porcelain base, then wipe the rod with a clean paper towel.
9. Reinstall the sensor exactly as it was, reconnect the wire, and replace the panels. Most furnaces will not run with the door off because of a safety switch.
10. Open the gas valve, turn the power back on, set the thermostat to HEAT, and watch a full cycle. The burners should stay lit.

Check whether the porcelain base is cracked or the wire connection is loose or corroded. A sensor that is badly pitted should be replaced with the correct part. If the furnace still shuts down, the cause may be a poor ground connection, a weak flame, or a problem with the control board. Short cycling can also come from a clogged furnace filter or a blocked condensate drain on high-efficiency furnaces, so check those too.

If you smell gas at any point, do not touch light switches or appliances. Leave the house and call your gas utility's emergency line from outside (Enbridge Gas in most of Ontario). Cleaning the flame sensor is routine maintenance, but in Ontario any work on gas lines, valves or burners must be done by a TSSA-registered gas technician. Ontario law also requires working carbon monoxide alarms outside every sleeping area in homes with a fuel-burning appliance, fireplace or attached garage; test yours when you start the furnace for the season.

Call a licensed HVAC technician if the furnace still locks out after cleaning, if you see a cracked sensor or damaged wiring, if the flames look yellow, lazy or lift off the burners, if your carbon monoxide alarm sounds, or if you are not comfortable working inside the furnace. A yearly fall tune-up, which includes cleaning the flame sensor and checking the heat exchanger and venting, prevents most of these mid-winter shutdowns.