Remember the tab marked Manuals has Lennox guides to help you access the flame sensor for cleaning.
Does you furnace act like it is going to start up, but then shut down? Does it do this a few times and then stop until you cycle the power off and on? If so, you're likely having flame rectification problems
The flame sensor is a rather simple device located at the burner assembly. It’s not much more than a thin, usually bent, metallic rod that sits in front of the flame stream inside the furnace usually in front of a burner opposite the igniter. In a case of a pilot with spark it sits in the pilot assembly. With most brands the lead is usually white or yellow in color.
The purpose of the flame sensor is to confirm a flame is actually present. When your furnace begins to starts and the burners are ignited, the flame sensor has a very short time to detect the flame. If the sensor doesn’t detect any flame, it will stop for a period of time and retry, five times, then shuts down. Most units have watchgaurd, which allows the Unit to cycle and start again after 1 hour, or power recycle from the main power or the thermostat.
It does this by Flame rectification. A small amount of current is sent down the wire and when it touches the flame (a polarized gas) it is rectified (half wave rectification). The control device will look for DC micro amps of 1.5 (.0000015 amps), unless your meter is equipped to measure this low accurately a cleaning or replacement is an equally good test.
Dirt from carbon, dust etc… can buildup. Because a flame sensor has a very low tolerance for variations in the reading it takes, the slightest coating can cause it to misread and shut down. 80% Units (1 pipe out with non sealed combustion) use all of the need air for combustion, primary and dilution from the indoor air. Indoor air has all the indoor pollutants from aerosols, cleaners and everyday human activity.
Shut off the Power to the Furnace and turn the gas off. Remove the Flame Sensor, it is typically mounted by one 1/4" hex head screw. Once you have removed the sensor, gently clean the metal rod with steel wool, a nylon scratch pad (like found in a kitchen) or a last resort a very light grit sandpaper (the silica can coat the rod).