I have a Bryant gas furnace, model# 310AAV048135. It comes on, runs for a couple minutes then shuts off while the blower still runs. I am getting a fault code of 33, which indicates several things according to the label on the device. Earlier in the summer, I was having a similar problem when the AC runs it was cycling too often and the service tech said the Draft Safeguard Switch could be faulty, since he could not find an obstructtion in the vent. It was not real bad, so I didn't do anything at the time. Now with temps reaching sub-zero, it seems to be more of a problem. Is there anything I can do to fix this without an expensive service call? Or is this something which needs the attention of an expert?