Many Lennox furnaces feature an electronic display panel with controls for the furnace, a window to view the pilot light and a small LED light that blinks when something has gone wrong with the furnace. However, identifying what the blinking means can be difficult without an understanding of the different flashes and what the flashes signify. If the LED light is blinking three times, this means that there is an issue with the ignitor or with the pressure switch.
Turn off the Lennox furnace and allow the furnace to cool completely. Open the front access panel.
Examine the connections at the ignitor and tighten any loose elements.
Check the gas supply valve and open the valve completely in a counterclockwise motion to ensure that there is a full gas supply to the furnace.
Carefully use a soft brush with a long handle to clean away any dirt or debris that may be clogging the ignitor.
Check the vents around the furnace and remove any obstructions, then remove obstructions from registers throughout the home and open all vents to allow for improved airflow. Note that closed vents in the home while the furnace is running can create a pressure imbalance, forcing the pressure switch open.
Open the interior doors in the home for unobstructed airflow, as closed doors also alter the furnace pressure.
Set the thermostat to the lowest point and adjust the gas valve clockwise to the "Off" position. Allow at least five minutes for any gas to exit the lines; stop immediately if you smell gas. Leave the house and call your gas company. Turn the gas valve counterclockwise to "On" if you do not smell gas after five minutes, then close the access panel. Set the Lennox furnace power to "On" and set the thermostat to the desired warming temperature.
Find the red reset button on the main access control panel of the furnace and depress it. This will reset the pressure in the furnace.