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Hello that is a pressure switch code, Remove both inspection panels. You will see a push switch where the top of the bottom cover rests on the furnace frame. Hold this button and see what happens. If the small fan in the top section runs and then shuts off you can bypass the sensor to rule out the ignition and gas issues. The sensor is round with two small tubes going to it. There is one wire going to the top and two wires joined together going to the bottom. You will need a test lead or a short piece of wire to do this troubleshooting. Put the bottom cover on so the push switch is activated. Remove the two sets of wires from the pressure sensor. Once the small fan is running joint the two sets of wires with the test lead or electrical wire. Look in the little window at the burner box and see if it glows orange (this means your ignition is working) the gas should light and burn blue and give you heat. When the house gets to temperature remove the test lead as this only works once (the furnace will not light with the wire joined - the small fan has to run with the wires separated before the startup sequence will work). The hoses going to the pressure switch will need to be cleaned out to get the furnace working properly but at least you can do it. You may also need to remove the pressure switch to look in it to see if the passageway is blocked. If there is no blockage in the hoses or passageway the switch itself may be defective.
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