Of course coldest night this fall and the furnace isn't working right. Had some work done in he basement and the air exchange pipe coming off our gas furnace was routed to the north side of the house next to the exhaust pipe. When this was done the exhaust pipe was also cut back towards the house at a angle for some reason and the elbow I had in place removed that angled the exhaust down and away from the house. Now tonight we have had strong north winds and the furnace wil start up and run a bit but then cycle off. We had the same problem when the exhaust pipe Iced up last year because of a pest screen that was on it. I checked it and its clear so the only variables are the slant cut or the relocation of the exchange pipe. Also have not noticed this problem yet this year so is it possible it's being caused by the wind and the exhaust pipe no longer being angled down? Any help is greatly appreciated as we Are down 6
Degrees already tonight

most newer furnaces have a small LED that will flash a code when a fail to fire occurs. The code key should be on the inside of the front cover. Next time is does it check the code and it should tell you exactly what it wrong. Could be  exhaust back pressure caused by wind, had that happen once when it was really windy. Most will lock out for a couple of hours after 3 failed attempts too.

Could be a problem with the flame confirmation circuit or gas valve too. Is the burner actually firing or is it just cycling and not firing? Could be the solid surface ignitor if it is cycling but not firing.  Any way to return the inlet and exhaust to the way they were or as close as possible.

J-

I'm not an HVAC guy but have fixed mine a few times. Make sure that when it cycles it is actually firing the burner. I had a problem with the flame detector and it would turn on, run for about a minute normal but the flame detector would not sense a flame so it would shut back off. I took it out and cleaned it with a little bit of steel wool and put it back in, worked fine and now do it once a year to prevent it.

I also had a problem with my ignitor, it is kind of a glowplug style ignitor and had a crack in it preventing it from working. The furnace would turn on, run the pump for a few seconds but the ignitor never glowed, gas would click on for a second and once again no flame detected so it shut off. I used a grill lighter to fire it up and get the house warm until I could get the part the next day.

It sucks in that nobody wanted to sell me just the part. They wanted a $100 trip charge for me to hand them the ignitor and tell them "this is what I need" All the supply places claimed it was a liability issue that they couldn't sell it to me but bullshit, it's protecting the profit of those who do a lot more business with them than I do. I could have ordered the part online and wait a few days to get it but I called my wifes uncle who knows some guy who does HVAC work and he ordered it into a supply house for me and had it the next day.

Good luck

What are the run lengths? I'd look at the flame sensor as the first area to check, then I'd pull the fresh air intake off the furnace and see if it works correctly. If it does, then that's likely the issue. Remember, if the intake/exhaust air flow isn't enough, it won't allow the inducer to make enough pressure [or vacumn, depending on the make] to close the pressure switch and power up the ignition circuit. You can also hook up a ohm meter up to the pressure switch and see if it's dropping out momentarily, all it takes is a split second and the computer will start the process over or lock out after several tries.

I'd still go with the flame sensor first though, especially if you are on propane.

when I get home I'll post a link to where I buy mine on line. Usually buy them in lots of 3 for about 60bucks shipped to my door. most furnaces use the same solid surface ignitor.

I usually have to change mine out every 1.5-2 years when they crack. Big thing is make sure not to touch the surface of the ignitor with ur hands/fingers, the oil can shorten the life of them appearantly.

J-

ETA: here is the link (think this is were I got the last ones from) http://www.theignitorstore.com/catalog/furnace_igniters?gclid=CO6605WK86QCFZk65QodmXvtfw

When most furnaces start, the exhaust fan starts first and a pressure switch activates once the exhaust fan gets a suction going (by the bernoulli principle).  If the wind blows into the exhaust pipe sufficiently hard, it will destroy that suction, the pressure switch won't activate (or will switch off) and the furnace shuts down.  But that'd have to be a heck of a wind and it'd have to be blowing right into the exhaust pipe.  

edit to add:  just re-read your post and this might be the problem.  Try putting a piece with a 90 degree downward angle on the end of the exhaust pipe.