Pressure switches are one of the most misdiagnosed components on a furnace. Their are multiple scenarios where a gas furnace can cause a pressure switch not to function. First you must know what a pressure switch is made of. The switch contains an elastomeric diaphragm that is connected to an electrical circuit. The diaphragm remains in the open position while the system is off. When the furnace has a call for heat from the systems thermostat, the draft inducer motor starts up causing the system to pull a negative pressure. When the systems designed pressure is achieved, the pressure switch will close causing the low voltage circuit to be completed and the next stage of the heating cycle begins. There are a few scenarios that will cause the pressure switch not to achieve its designed pressures:

• A bad pressure switch (bad or pitted diaphragm)
• A weak Draft Inducer Motor
• Crack in the Heat Exchanger and or Draft Inducer Housing
• Blocked or bad pressure switch hoses
• Clogged internal or external drain lines (Condensing Furnaces)
• Restriction or improper sized vent pipes
• Lack of combustion air

Before you can come to the conclusion that the switch is defective you must first rule out all possibilities listed above. A "quick" fix that is often used is to simply change the switch, however it is rarely the pressure switch that is problem. This is where a technician needs to bypass the pressure switch to further diagnose the furnace. Bypassing a pressure switch is not the easiest procedure to do. You must be able to complete the circuit in time for the control board not to sense an open or closed circuit. The general practice when jumping out a pressure switch is to remove the circuit from the pressure switch, place a jumper wire in one end of the connecting circuit. When the draft inducer motor starts, the other end of the wire needs to be connected to the other circuit wire before the control board recognizes an open pressure switch. If this connection is made, the system will proceed to the next step. While this process can be tricky you must also be sure the wire does not touch anything which would result in the system shorting out.  

The Furnace Bypass Switch Diagnostic Tool would simplify this process for you.  Simply remove the the circuit leads from the pressure switch, hook them to the Furnace Bypass Switch protected connectors.  Once the draft inducer motor starts simply push the toggle switch to connect the circuit. An LED indicator will light up when the connection is made, also confirming low voltage is passing through completing the circuit. While this switch is in place you are able to run the system to rule out all of the issues listed above. Once the issues listed above have been ruled out, you are now safe to assume the pressure switch needs to be replaced. The Furnace Bypass Switch will need to be removed.  Leaving the switch in place could be dangerous and may result in fatal injury.  

If the LED indicator does not light up once you have made the connection then you will know that you have a voltage issue that will need to be addressed before continuing to test the pressure switch.