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There is a draft fan in the upper section attached to vent pipe. Then you will see a rubber tube as thick as a pencil, this runs from the furnace to a small pie shaped switch with 2 wires attached. ill look like the switch at this link.
The exhaust fan is the inducer motor. The rectangle behind the gas valve with 2 wires is the limit. We will be checking that also. To test the pressure switch...
remove the 2 wire from the switch (not the limit) hold them apart, start furnace, when inducer fan kicks on touch them together so the metal ends touch and hold them that way. See if unit progresses
If the unit ignites when the 2 wires are touched = cracked rubber hose, plugged nipple port where the tube attaches, plugged vent, bad pressure switch.
If it does not change the code and we still are at same fault, look around burner area for dime sized switches with little reset buttons in the center. Also remove the 2 wires from the limit switch on the rectangle plate and attach together so metal ends touch. Then let me know what has happened. I will be online tonight
Thanks for the information. I was not able to work on my furnace last night. it was running just fine. I will wait for it to fail again to run the tests. There is a storm coming in this weekend and temps will drop. The way things are going right now, it will probably fail when I need it most. Luckily, I have two furnaces that heat my house so it won't be quite so bad. I will let you know how it goes.