I have a Lennox g40uh series furnace and it just stopped working today. I can reset the furnace and can see that on the control board both LEDs (green and red) are blinking together which indicates the furnace is operating correctly. I can also hear a small click when I reset it as well. After a minute or so the red LED turns off and the green LED blinks. That's it. Nothing else is happening, no inducer fan, igniter or blower is turning on. The diagnostic sheet gives the following for the blink pattern:

(A) Pressure switch open or,
(B) Blocked inlet/exhaust vent or,
(C) Condensate line blocked or,
(D) Pressure switch closed prior to activation of combustion air inducer.

The inlet and exhaust vents look good, nothing seems to be blocked. I can take off the small rubber tube going from the inducer to the pressure switch and use a straw by blowing/sucking to open/close the switch manually, that should eliminate (D). Normally the inducer will start up first and will create a negative pressure to close the pressure switch but since this isn't happening I am assuming this is the problem since the control board doesn't detect the pressure switch being closed, the igniter and blower won't turn on.

Does anyone have any other ideas I can try? My guess is possibly a bad control board that is not turning the inducer motor on.

The furnace was off all night but sporadically came on around 6:00 AM this morning.

4 Answers 4


Sounds like you have most of the common bases covered. This happened to me and it was the blocked inlet valve, a leaf had gotten sucked in. It took me a while to find it, it was not in the visible area of the inlet, it was stuck on a screen to catch debris. I would double check that. Maybe try to blow some air in the inlet from the outside of the house and see if you feel the movement in the furnace.

This question is similar and may have some other suggestions.

  • Thanks for the tip, I will do a more thorough check on the inlet and see what I can find. Commented Feb 1, 2011 at 15:35

Finally got it fixed, the control board was malfunctioning. After getting that replaced the furnace fired up right away. The most likely culprit for the board going bad was dust and dirt when the basement was finished 3 years ago from all the sheet rock. Take care when putting up sheet rock, it is very hard on the furnace.


I have this same furnace, but the attic version. Ours started acting up sporadically - the thermostat would click, the furnace would power on (start the first blower), it would light the pilot light and then shut down. According to the diagnostic LEDs, it was failing in the flame sense step.

What ended up fixing the problem was cleaning off some build up on the flame sensor (for me, a short metal rod that sticks into the middle of the pilot light). All I had to do was give it a quick rub with a bit of steel wool, and the furnace was as good as new.


My Lennox SL280UHV was coming on slow speed, ignitor and burner would operate and burn gas for about 4 seconds, then shutdown. Orifice on Pressure Switch to Draft Inducer Clogged with carbon, Bored out orifice and Vacuumed up the Carbon.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.