Very funny. I have seen similar things before. IT is always the last thing you think of.I have been struggling for a week along with my HVAC guy to figure out what was wrong with my York. I had the exact same symptoms as PDX...furnace 3.5 years old, and it kept going to the 3-flash, pressure switch stuck open. Every once in a while it would give me a 6 flash or an 8 flash, but they all pointed to the pressure switch. The HVAC guy even replaced the pressure switch last week, but the problem persisted.
I woke up at 4:30 this morning and the house was 59 degrees (Minnesota). I started turning the furnace off and on trying to override the failure just to get the house warmed up and that was when I got online and found this discussion. I appreciated some of the good suggestions, but I can say that we are now 100% sure we have fixed my problem and I wanted to share it.
I have had a field tech coming almost every day for the last week and he just hadn't cracked the nut, so this morning the owner of the firm came out too. What the owner knows (because I have known him for years), that the field tech and you all don't know, is that I have a 2 year old boy who likes to be a stinker. The owner showed up and didn't even bother ringing the bell. Instead he walked to the back of the house, pulled the PVC venting loose at the first corner where it came out of the house, and the problem was solved. My boy had been dropping Class V rocks down the vent all summer and it was choking off the intake! It would be like trying to run a marathon with a mouthful of marbles!! Glad to have the problem solved and thought I would share it with all of you. Enjoy.