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This is a draft pressure switch error (see attached pic for typical). Is the draft fan running? If not then turn the power to the furnace off wait a minute then turn it back on again. If it is running then the first thing to check is for a blocked vent. If snow is blocking the exhaust vent termination this error will display. Also check the small vacuum tubing connected to the switch. If the tubing is kinked or has water in it that will cause this error.