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If the nipple isn't clogged, pull off the hose completely and check it for obstructions. Also poke the nipple at the sensor. If that doesn't work, pull the combustion air duct off the top of the unit and make sure it's drafting. You should feel air turbulance at the duct opening. If it's not drafting, you probably have a bird's nest or leaves stuck in there.
If it's not the air sensor, then it may be something serious like the control module. Run the furnace and let it shut down of it's own accord, open up the combustion chamber on the bottom and check the LED code. It will probably be blinking, or perhaps have an error code. Check this against the troubleshooting guide in you owner's manual.
The unit will not run with the combustion chamber open, so you will have to put the access panel back in place to see if its working.