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please refer to the diagnostic chart in this manual - http://www.cozyparts.com/Lennox-Parts/forum/Lennox_service_manual/Furnaces/G40UH_G40UHX_09-01-2006.pdf
Please tell me the fault it matches.
What does your code translate into? Pressure switch open when should be closed....limit...roll out open? Flame failure?
The burner area is guarded by small round switches with a tiny button in the center. They will be on top or side of burners. Press the small center button and see if switch clicks.
There is no code for same time fast flash for both...they are not alternating?
I saw that just now that you mentioned that.The lights are both flashing at the same time and speed.That shows in the manual as normal operation, thats why I thought that it might be the flame sensor that had gone out or ignitor..
So at this point then no exhaust fan runs, burners, blower...nothing?
No sound or operation?....this would make sense
This will have to process of elimiation...
are you familiar with pressure switch?
See page 14 for combustion prove switch.
It has a small rubber tube attached to it from the exhaust motor.
turn furnace off....remove the 2 wires from the prove switch
start furnace....AFTER the exhaust fan starts touch the 2 wires together so they make metal to metal contact. and hold them that way....tell me if unit shuts down at 3 min again.
Nope...if tube long enough trim off cracked end
Ok...just for testing purposes....
do the wire test I described.
Great job.....I admire your persistance,......many give up.
You are very welcome....enjoy the rest of your weekend!