I have a YORK furnace - Ultra Diamond 95 installed in 1996. It is a variable speed two stage furnace rate 112, 000 Since day one we get it comes on runs 20 secs and turns off. We have had a number of service persons come out to check and no success. Finaaly, I spoke to York and they sent out a tech who checked the following: 1) intake and vent pipes for blockage 2) control board 3) pressure switch 4) ventor motor 5) condensate drain He said all was fine -- left the furnace running and of course it stopped as soon as he left. But he did recommend changing both the intake and vent pipes to 3inches. They are 2 inchees now. Should this be done? The quote was $300. Thanks