Lennox furnaces with these two model numbers have a intermittent problem with them in the way that they will run fine for a long time, Then all sudden stop working and give you an error code of pressure switch open . But when a professional shows up to repair the furnace they find nothing wrong . The issue I have found many times is when they manufactured the venter motor assembly they painted it black, Paint chips would flake off make their way to the pressure switch and block off air flow . To fix this problem you would need to replace the motor assembly, pressure switch, and vacuum lines .

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