Okay, first off I have a Lennox Furnace model # G14Q3-60-20 serial # 5890k00091. I replaced my motor last year on it and it was working fine until I went to turn it on this year! It will click on then go right off. After speaking with a guy who works on Lennox Furnaces and he told me that he was 99% sure it was the flame sensor and to take it out and clean it since that is usually the problem 9 times out of 10. So after a while of trying to locate it I found it and cleaned it like they said, and it is still doing the same thing! Please Help!!

A G14 is a pulse furnace. You are pretty good if you were able to remove the flame sensor,they are pretty tricky to get out.

I need a little more info, how long does it run once it starts to rumble? If it shuts down in a matter of seconds after lighting,most likely you will need a new ignition control.