The above photo shows one of the problems I encountered one winter when our furnace stopped working. It would start, run for 30 seconds, and then shut off. (Basically tried to start the heat, but never turned on the burners.)

I checked the LED status indicator and it told me it was a pressure-sensor code. I opened it up, and found the pressure-sensor pretty easily.

I tried to start it and noticed heard the water sloshing around in the fan-housing as it tried to start, and noticed a little bit of water underneath the fan-housing. The water was obviously blocking the pressure-sensor hose and causing the code to trigger, thereby shutting off the furnace.

The problem turned out to be a blocked condensate drain-hose which caused the condensate water to back-up into the exhaust housing and overflow into the fan-housing. (the design is obviously setup to prevent this under normal conditions).

A friend called a HVAC person he knew and the guy said this was a “known” problem. I pulled the drain hose off the bottom of the exhaust housing chamber, and blew it out w/ my mouth (heard a distinctive “pop” as it became unplugged), and the problem was fixed.