I have a bryant 80 plus furnace that is about 12 years old. It is giving me an error code of 12 which is blower on at start up after a call for heat. The blower runs for about 90 seconds then shuts off. It then gives me an error code of 31 which sent me straight to the pressure switch. When I checked the pressure switch for voltage I had none, so I changed it out and I'm still getting the same code. Any ideas?