How to Clean a Furnace Flame Sensor

What the Flame Sensor Does

The flame sensor is a small metal rod mounted in the burner assembly. When the furnace fires, an electrical current passes through the flame sensor. The flame conducts electricity (a principle called flame rectification), and that conductivity confirms to the control board that the burner is lit.

If the sensor is coated with oxidation, the current can't flow, the control board doesn't see confirmation, and it shuts the gas off as a safety measure. The furnace may try again two or three times before locking out entirely.

What You'll Need

• Screwdriver (usually 1/4" hex or Phillips, depending on the furnace)
• Fine steel wool or light sandpaper (320-400 grit)
• Clean dry cloth

Do not use heavy sandpaper or abrasive pads that would remove metal rather than just the oxidation layer.

Step by Step

1. Turn the furnace off. Turn off the furnace at the thermostat and at the power switch on the furnace cabinet. Wait 5 minutes.

2. Locate the flame sensor. Open the furnace access panel. The flame sensor is a small metal rod, about 2-3 inches long, mounted on a bracket near the burner with a single wire connection.

3. Disconnect the wire. The wire connects with a push-on connector. Simply pull it off.

4. Remove the sensor. There's usually a single screw holding the sensor bracket. Remove it and slide the sensor out.

5. Clean the rod. Lightly rub the metal rod with fine steel wool or sandpaper. You're removing the white or grayish oxidation layer. The rod should be bright and clean after a few seconds. Wipe with a clean cloth.

6. Reinstall. Slide the sensor back in, secure the screw, and reconnect the wire.

7. Restore power. Turn the power back on and turn the thermostat up to call for heat. The furnace should fire up and stay lit.

If It Doesn't Work

If the furnace still short cycles or locks out after cleaning, the sensor itself may have failed and need replacement. Flame sensors are typically $20-40 for the part.

There's also the possibility the issue isn't the flame sensor at all. A furnace that cycles out after cleaning the sensor may have an ignition problem, a gas pressure issue, or a control board problem. At that point, call a technician for diagnosis.