On Tue, 13 Jan 2009 13:05:44 -0800 (PST), "
wrote:

A little help here.. I have a gas furnace about 13 yrs old, and it
runs most of the time. the times it doesnt it seems that the pressure
switch is not working. i would take the little hose off it for a
second and then it turns on. i replaced the switch with another one,
not exactly the same it was .30 vs .35 plumbing store said it was
close enough, but that didnt make it go on. what else could be the
problem? is it bad to keep the hose off so it works everytime?
the furnace guy tried to charge me over 1000 total, 100 for
diagnostic, 470 for ignitor, (which works) and pressure switch another
470, which i purchased for 40. some garbage huh?

Our 5-yo high efficiency gas furnace quit working recently, and the
cause was a slight sag in the exhaust PVC piping run (horizontal out
the side of the house) that apparently was not exhausting the fumes
and kept the pressure switch from activating the fuel (or something
like that). All is well now, no parts were replaced.