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A 31 code is an open draft pressure switch. Is the draft fan running? is the exhaust completely clear of any obstructions?A 12 code is unit powered up during a call for heat which would happen if you powered the unit down to try and reset it
This fan creates the draft that gets rid of the gas exhaust. It the small fan on the front of the unit. The exhaust vent begins at the draft fan.
Do you have a multi-meter to test voltage?
Ok, once you have it check for 120 volts AC to the draft fan after you cycle the power off then back on again.
You can't find what?
Then you can't troubleshoot this any further. Most likely culprits are a bad fan (motor) or a bad control board. Since each of these is pretty expensive you need to be sure which is faulty. The only other thing you can do is check the electrical connections to the draft fan.