A month or so ago, I woke up to discover that I had no heat in my house. I went to the furnace to do a diagnostic check, removed the service door and quickly discovered that there was no flame (visible through the glass peep-hole).
I followed the manufacturer's shut-off procedure, which can be found in the manual, here: http://legalectric.org/f/2011/03/carrier-9200-weathermaker-uhe-om58-67.pdf
. When I turned the furnace back on, I saw that the igniter did not glow (should be visible through the same peep-hole - should look like a red-hot, 2-prong tuning fork) and that the red LED error code indicator (visible through the lower peep-hole) was blinking 3 short flashes and 1 long flash (error code 31).
Now what I SHOULD HAVE done was check the pressure switch first (see service manual pg. 3, component #16). By removing the rubber lines that connect tot the pressure switch, one end at a time and both at the pressure switch and the bottom of the igniter box, you may notice some debris in the lines or the male adapters on the pressure switch or igniter box. If you do, clear out the debris with a paper clip (don't insert the paper clip into the body of the pressure switch, though). After checking all connections for debris, disconnect the hose that connects to the top of the pressure switch at the "T" between the pressure switch and the igniter box and blow/suck air in and out of the pressure switch (lightly). If you hear a small click when you pass air in and out, that's the pressure switch functioning. If you hear crackling or you can't blow in/out easily, you probably have a bad pressure switch.
if you are able to make the pressure switch click, connect all lines and start up the furnace per the mfg recommendations. If the furnace kicks on, it was probably a blockage in the pressure switch lines (likely if you found gunk in the lines or male adaptors on the pressure switch), but it may still be a faulty pressure switch (my problem... when I blew in and out to the pressure switch, I could hear the silicone diaphragm cracking). I got mine going, albeit temporarily, by removing the pressure switch and passing air in and out (blowing pretty hard) with my lips on the bottom hose adapter until I could hear the diaphragm crackle free and it became easier to make it click on and off. That only bought me a few days, though. After that, I bought a replacement pressure switch at a plumbing supply place and replaced the old one. If you found gunk in the lines, I would suggest you have your furnace serviced and let the tech know what you found and where.
If neither procedure fixes the problem, it may be the igniter. For some odd reason, this issue started for me about a month ago and I replaced the igniter (YouTube video instructions are easy to find), which fixed the problem. If even that doesn't work, call a local tech, shut the gas off and wait around in some thick blankets! I hope this helped!