warm the house with the kitchen stove temporarily with the window open
an inch or 2.
i looked for your mfg on google.
i might try to register at mfg website and check there for possible
tips and manuals at:

http://www.lennox.com/owners/
you have 5 to 20 year warranty on these

i dont know your furnace, maybe updraft flue blower tube off motor
needs cleaning with a paperclip is a common problem on a couple of
mine. which means if you pull off the hose from the flue blower and
sucked on it would the switch close and allow furnace to operate.