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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22541
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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I have a lennox G40UH x series furnace that will not light,both

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I have a lennox G40UH x series furnace that will not light,both the red and green light on circuit board are flashing fast at the same time.Would this be the flame sensor needing to be changed.The unit is about 4 years old.....





please refer to the diagnostic chart in this manual -


Please tell me the fault it matches.



Customer: replied 6 years ago.
I have the manual with the codes for trouble shooting,its a simultaneous fast flash,both green and red light,the air inducer comes on like it should,but no flame in gas chamber.


What does your code translate into? Pressure switch open when should be closed....limit...roll out open? Flame failure?







Customer: replied 6 years ago.
I believe its the roll out.


Thank you,


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.



Customer: replied 6 years ago.
I pressed both of them,didnt feel or hear a click on them,still no flame.


There is no code for same time fast flash for both...they are not alternating?



Customer: replied 6 years ago.

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

Customer: replied 6 years ago.
I am stumped with this thing,everything fires up like it should,the combustion air inducer comes on, but no flame,could it be the gas valve not putting out any gas,I already tried turning it on and off a few times..and yes no blower or burners come on at all..the air inducer stays on for about 3 min then shuts off ...


This will have to process of elimiation...


are you familiar with pressure switch?



Customer: replied 6 years ago.
I am not,would that be the one on the air inducer?


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.



Customer: replied 6 years ago.
I just saw the hose,its cracked at the you still want me to try the wire check ?


Nope...if tube long enough trim off cracked end



Customer: replied 6 years ago.
It is not long enough,and I dont have any small tubing to use on it..



Ok...just for testing purposes....


do the wire test I described.



Customer: replied 6 years ago.
I used a piece of tubing from my new fluid master I have,I'll try that real quick...if it still does the same ,I will try the wire test....



standing by....



Customer: replied 6 years ago.
Thank you Billy !

That cracked little hose was the problem.The furnace came on and stayed on till it reached the setting on the thermostat.....Thanks again...Alex

Great job.....I admire your persistance,......many give up.


Well done!



BillyHvac and other HVAC Specialists are ready to help you
Customer: replied 6 years ago.
I was so happy you were online...Thanks again Billy, I couldnt have done it without you..


You are very welcome....enjoy the rest of your weekend!



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