It's June so my furnace has been off for a while. But it's cold today and I have a sick kid home so I tried to start my furnace but it's not working. I'm getting an LED code with three red flashes which, according to the label on the inside of the access cover, means "Vent intake or termination problem". I noticed that the filter (16 x 20 x 1) was out of place and kind of pulled a little into the unit. But it didn't look like it damaged anything. It was also really dirty, so I figured maybe the issue was that the dirty filter blocked the air enough that it triggered the error. So I bought a new filter and replaced it.
I couldn't find a "lockout reset" button on the unit so I reset the LED fault indicator and turned the unit off for a minute or so, hoping that would reset it. That didn't work and I'm still getting the 3 red flashes. I waited an hour, thinking that it might reset automatically (which I know is the case with some units) but that didn't work either.
Any suggestions as to what else I can check? The intake where the filter goes looks fine; as for the outgoing vent--I don't see how I can check that for a blockage. It vents through a metal vent into the chimney. I also don't understand how that would have gotten blocked up.
The unit is a York TG8S gas furnace. I haven't been able to find a service manual online--just a "technical document" that wasn't much help and an owner's manual that was pretty useless from a technical point of view.