Furnace Flame Sensor: Cleaning Guide and Signs of a Bad Sensor

The flame sensor is a thin metal rod that sits in the path of the burner flame. Its job is safety: it confirms a flame is actually present so the furnace does not keep pumping gas without ignition. Over a season or two, the rod develops a coating of carbon and oxidation that stops it from sensing the flame. The furnace lights, fails to confirm the flame within a couple of seconds, and shuts the gas off, then tries again in a repeating cycle. A dirty flame sensor is one of the most common no-heat causes, and cleaning it is a manageable homeowner task.

1. Turn the furnace off at the thermostat, then switch off the furnace power switch and the gas shut-off valve.
2. Remove the furnace access panel. The flame sensor is a thin metal rod near the burners, held by a single screw with a wire attached.
3. Remove the mounting screw and gently pull the sensor out. Take a photo first so you can reinstall it the same way.
4. Lightly clean the metal rod with fine steel wool, an emery cloth, or fine sandpaper. Do not use a coarse abrasive; you only want to remove the film, not scratch the rod deeply.
5. Wipe it with a clean cloth to remove dust, then reinstall the sensor and screw, and reconnect the wire.
6. Replace the panel, restore gas and power, and run a heat cycle. The furnace should now light and stay on.

Cleaning fixes most flame sensor problems. If the furnace still short-cycles after a careful cleaning, the sensor itself has worn out and needs replacing, which is an inexpensive part. Repeated flame-failure codes after cleaning and replacing point to a different issue, such as a weak gas valve, poor grounding, or a control board fault, that needs a technician.