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Lennox G40UH-60D-135-14 Furnace will not ignite

\015 Everything seems to be in working order except the furnace will not fire up. I checked the thermostat, the fan will run on auto and manual. This happened out of the blue with no warning. What is going on? Do I need a service call or is this something simple that I can do myself. Any advice would be greatly appreciated. Thanks\015

Answers :

Do you have an inducer on the unit (no access to the lennox model number tonight)? That is a motor that comes on shortly before your unit fires that blows the combustion gases up through the flu. Does this motor come on at all if you have one? If it does and the furnace will not light, then there is a device on the unit called a pressure switch. This switch has two wires to it and a small tube that leaves it and goes to the inducer housing. Remove the tube from the inducer and blow into and draw in air lightly or until you hear clicking noises. Replace the hose and try again. Sometimes the pressure switch gets stuck but can also go bad. If everything is coming on all the way up to ignition but the unit won't come on, then it's probably one of the limit switches that need to be reset. These are little disks about the size of a penny and have two wires going to it. In the middle is a small button that can be pushed to reset it. Some units have 2 to 3 and others have more. You have to use a flash light and look all in the unit around the burners and around the flu pipe looking for the limits. Once you have found them all and tried to reset the unit, then try to start it up again. Hope this helps.
I had a similar problem that started right after i changed the batteries in my Honeywell thermostat. I know this is really dumb, but it had me scratching my head thinking my furnace/pilot light was having issues. When i plugged my thermostat back into the wall, i accidentally bent the far right pin and it wasn't connecting (or was also touching another pin). This caused the same symptoms where everything seemed to be turning on when it was supposed to, yet the heat never fired. Frustrating, but in case anyone just changed the batteries, you'll want to make sure it fits perfectly back on the wall and that you didn't bend a pin.

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