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The code means that the vent motor is running but there is no pressure differential being sensed by the pressure switch. The pressure switch is a device like the one in the photo,
If you remove the 1/4" hoses on this and clean out the barbed fittings that the hose (s) attach to you may get it to work, this device is there to ensure that you exhaust vent is clear before firing the furnace.
Is the vent motor running ?
Let me know,
That makes sense , unfortunately its not the repair we wanted.