One more question,
You mention it went back to not functioning properly. Is it throwing any codes similar to those that led you to check the vent pipe? Reason I ask is, if the exhaust bent is drooping, after a couple of cycles it will collect enough condensation to cause a blockage in the vent which would prevent the pressure switch from closing.
FWIW, I might be heading down to Washington Courthouse later in the week. If I do and can be of help, you have my number.

+1000
Maybe even exponentially.

You have been far more than generous. Maybe we could meet somewhere for coffee while you're out if the timing is right. I have met a couple of pelicans, can't hurt to meet another one.
Yes, it's showing the same code, three flashes. That's probably a good diagnosis. I love the challenge of wrestling a problem to the ground, and obviously you do too. PPOT is a great community!

stupid question, i'm full of them, have you changed the filter?

LOL!
Yes. And there are no stupid questions. That's just the kind of thing that pounded that flat spot onto my forehead.

FWIW, The reason I mention condensation is because I had that very issue pop up on a system once. I strapped the vent to the proper pitch (it should pitch back to the furnace where the condensation will flow from the condensate drain) and the problem went away. If it was a circuit board issue I don't think that it would cycle a few times before showing up. I suppose it's possible for the IFR (inducer fan relay) to be loose causing it to intermittently work but I think since it worked right after you did the vent cleaning the first time I think it's something vent related. If you decide to try and pitch the vent you want at least 1/2" per 10 foot section sloping back to the furnace. Don't do a hard reset (on and off with the breaker) but rather turn the thermostat down so that the call for heat would end and then switch it off. This is called a "soft reset". The furnace should try to run after a soft reset unless it it locked out due to too many attempts and failures.

If you're willing to have a go at it, I'm willing to advise... And if by coffee you mean a beer, I'd be game. :D

Another possible issue that can cause an ignition failure (I'm guessing that's what he 3 flash code means) is a blocked drain. Typically where the small tubes connect to the collector box of the secondary heat exchanger, it will become plugged with water if the condensate drain isn't flowing due to a blocked trap or other obstruction. Another thing I found once was a slight calcium buildup blocking one of the small ports where the tube for the pressure switch attaches to the secondary HE collector.

A thought on the "rust" in the HE and burner. I'm guessing you're burning propane. If I'm right, it's not all that uncommon for some rust in the Burner and HE. The burners on my old furnace had a lot of rust in them. My issue with many "technicians" is they don't troubleshoot anymore (listen to me! I'm all of 47 talking like the old guys LOL) If your combustion is a concern a good tech should carry a handheld Combustion analyzer. I haven't done HvAC for over 10 years and I own one because I don't like guessing. Analyzing the combustion will give a tech a good idea of the condition of the Heat exchanger and the overall condition of the system. Being old enough to be considered "old school" I can normally look at the flame and tell but a CO Analyzer is a better way to do it.

So, I had a very similar problem the past 3 weeks, with a 9 year old Trane XR90 furnace. Throwing the 4 flash code which indicates a problem with the temperature limit sensor. So, I ordered a new sensor. Didn't fix the problem. After going through the vent checks I noticed that if I unplugged and replugged one of the wire bundles, or just mucked around with it, the furnace would work intermittently. Taking it off I could not see any broken solder joints, and without any experience at troubleshooting a circuit board, I found a brand new one for $125. Just installed it 10 minutes ago and the upstairs is already getting toasty :)

I'll plug the parts source I used- northamericahvac.com. Customer service was absolutely fantastic, super fast shipping, and they even took the limit sensor back and issued a full refund. In their words, "if it did not fix your issue, please return it and we'll issue a full refund. Thanks!" Really good communication via email.

He had an inspection camera and took video of the inside of the heat exchanger. This was last January when I had the house inspected. He noted the rust, but did not see any cracks. BTW we're burning natural gas.

Welll...

He took the furnace out this morning and found this:

http://forums.pelicanparts.com/uploa...1544714999.jpg

I'm glad I decided to replace this furnace instead of trying to fix it.
Kind of annoyed that neither of the guys who looked at it did a safety check on this 20 years old furnace, they just focused on the immediate problem. A whole lot of effort could have been avoided if they had given the rusty 20 year old heat exchanger more attention to begin with.

Start chewing some gum and you'll have that fixed in no time:)

That thing should have been burning really yellow with a crack like that!
PSA: Always have a carbon monoxide detector in your home childrens!

Yes. Nasty (not in a good way) crack. I have two CO detectors and my house and should probably have more (one in each bedroom).
The one positive thing about newer furnaces is that the combustion side of the heat exchanger is usually negative in pressure.
Good that you have the thing changed out.

When mine when out a few years ago could have done the fix route, but listened to my HVAC guy. Got a new one because it was way more efficient. Also ended up doing the install over a Memorial weekend. Told the guy to go fishin it wasn't going to get either hot or cold that weeked. He gave me so much off because of that, couldn't find anything on the internet that size for the price including the install.