Repair of a WhisperHeat by Lennox G20 furnace. There are three sections in this video in which the pilot assembly, ignitor, flame sensor, main controller and blower controller are replaced. See below for the times in the video where each of these repairs take place.

Be aware that in some locales it is illegal for anyone other than a licensed professional to repair a gas appliance. Check your local laws. If for any reason you don't feel comfortable or able to make repairs safely on a gas appliance, do not even try. Call a licensed professional for your own safety and that of everyone in your household.

A note on heat exchangers: Heat exchangers can develop cracks from normal use due to repetitive expansion and contraction of the metal. These cracks have the potential of allowing deadly carbon monoxide gas to mix with the heated air and get blown into your
home where it can threaten the lives of all living within.

While your furnace is apart check the heat exchanger for any cracks. If any are found, have the heat exchanger or the entire furnace replaced. Note that due to the fact that heat exchangers are difficult to inspect it is not uncommon for cracks to escape detection. For
this reason it is very important to have one or more carbon monoxide detectors in the home. I prefer to have two in case one of them fails. Some are battery operated while others plug in to an outlet and have a battery backup. Some have a digital readout that tells you the carbon monoxide level detected in the air. All of them alarm when the level becomes dangerous.

Description and location of the three sections of the video:

Intro: Warnings and cautions - 00:00

Section 1: Description of problems, replacing the pilot assembly, ignitor and flame sensor - 01:48

Section 2: Replacing the main controller - 26:05

Section 3: Replacing the blower controller - 32:02