The procedure for cleaning a furnace flame sensor varies from one model of a furnace to another. In general, cleaning a furnace flame sensor involves removing the sensor from the furnace and scrubbing the sensor with an emery cloth before reinstalling it.
Start by gathering a screwdriver and an emery cloth. To avoid electric shock, switch off the breaker that serves the furnace at the main power panel before attempting any cleaning. Next, locate the flame sensor usually found towards the rear end of the furnace, then locate the rod in the path of the burner. Using a screwdriver, loosen and remove the sheet metal screws that secure the sensor in place to detach the sensor box from its bracket. If the sensor is irremovable, you should clean it while it is in its position.
Clean the sensor by gently scrubbing its surface with an emery cloth to eliminate all dirt and debris. Sandpaper should not be used because it can gouge the sensor surface. Once the sensor is sufficiently clean, reinstall it into the furnace, then turn on the power to check if the sensor works properly. Check and fix any installation defaults if the sensor fails to work.