I got two calls about a month or two apart same home, same problem.
No heat call on a fairly new American Standard 90% furnace, I get there, pressure switch not closing, so I hook up the manometer and vent is blocked. I cut it and looked down it and find her kids(assumed) dumped lava rock into it. I remove the obstruction and test it out all is well. Called a few weeks later, go back same song and dance, but this time they got trickier, MUD! I poke and prod and run water through the drain into a bucket at the other end of the vent and all is well.