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This is a code that indicates a venting problem. Here is what you should check. Make sure there is no obstructions in the vent pipe. Make sure the vent fan is running. If the vent fan is not running check for 120 volt power from the control board. If no power from the control board, replace the board. If there is power, than replace the fan/motor. If everything is ok with the vent fan check the flue venturi for any obstructions or melting. Ensure that there are no loose wires and that the pressure switch is opening and closing properly. Check the tubes coming from the air pressure switch for kinks, melting and holes, make sure that the low pressure opening at the back of the unit is not blocked. This about covers everything this code could be tripped by. Also check that the condensate drain is not blocked as this can also cause funny annomalies.