A Flame Sensor electrode is located inside the burner tube. This is a safety device that allows the gas to stay on only if the flame sensor detects the heat from the flame. This flame sensor is located at the bottom of the burner tube.
This flame sensor is in the inside of the burner tube where the flame comes in to the fire box a few inches back from the end. It is possible for ashes to get into this burner tube and cause the flame sensor to fail. Blowing compressed air back into the burner tube from the fire box chamber can clean these ashes out and cause the flame sensor to work again. Compressed air in a can can be used as long as there is no fire or embers in the fire box. The propellant in these cans may be propane and could catch fire.
If this does not correct the problem, the flame sensor may need to be replaced. The following pictures will help show the steps to access the flame sensor for replacement.
Unplug the orange wire and remove the ignition module cover. Unplug the blue wire after the cover is removed. Unscrew the ignition module bracket so the ignition module can be moved.
Remove the 4 screws that hold the gas train to the front of the burner plate. This will allow the gas train to be moved to the side.
Remove the 4 screws from the corners of the burner plate. Separate the two burner plates with a screw driver. Pull the inner burner tube straight out.
The blue wire is plugged in to the flame sensor.
The flame sensor is held in place with a screw and nut. The new flame sensor will position itself correctly when the screw and nut are replaced. Be careful when inserting the inner tube back in to the burner when reassembling so the flame sensor does not get bent out of position.
Sometimes the function of the flame sensor can be improved by making slight adjustments to the air shutter on the burner blower. The small screw at the top holds the air shutter in place. The setting of the air shutter is important to control the proper ratio of air and gas but minor adjustments can be made to help improve the efficiency of the flame. Normal settings are between 1 and 2 on the numbered scale.