age of your furnace can be found in the 1st 4 numbers of the serial # . 1st 2 are the week of manufacture and 2nd 2 are the year.
you might have a plugged condensate trap. pressure switch could even be sticking closed
as for everyone else: Make and model of the furnace is very helpful for us online diagnostic teckies as well as ignition type. IE; pilot flame, hot surface ignitor or direct spark. That is if you know how to tell them apart
Yep - was indeed a plugged condensate trap. It would build up and then drain slowly, so after a while it would work again, until condensate built up. Working fine now - and the exhaust line needed to be reworked anyhow.
Thanks so much.