The Pressure Switch Is Stuck in a Closed Furnace While It's On

Accessing the Pressure Switch

• For safety, shut off the furnace at the thermostat, flip off the circuit breaker for the ignition and wait for the unit to cool before opening the furnace. Since the switch is inside the furnace, you'll need to remove the service panel from the side of the unit by taking out the bolts with a nut driver. Set the panel aside.

Locating the Pressure Switch

• On most gas furnaces, the pressure switch is mounted near the top, adjacent to the circuit board and wiring harness. The fastest way to identify the switch is to locate a flexible plastic tube that runs from the bottom of the furnace, where the blower motor is installed, to the point where the tube connects at the top. The pressure switch is the top connection.

Fixing the Pressure Switch

• The pressure switch operates with a plunger that slides in and out by air pressure from the flexible tube. If the switch is stuck, this means the plunger will not move. Access the plunger by pulling gently on the end of the tube where it connects to the switch. Spray a short burst of silicone lubricant, such as WD-40, onto the switch plunger and wait five minutes for the lube to work into the part. Pull and push the plunger repeatedly to loosen the mechanism. Disconnect the tube at the bottom and blow through it to remove any clogs then attach the tube to the motor nozzle at the bottom and the lubed pressure switch at the top.

Closing Up the Furnace

• Check inside the furnace to verify the two wires on the pressure switch are still attached to the switch terminals. The wires may become dislodged while working on the switch. Hold the service panel to the side of the furnace and bolt it in place with the hardware removed earlier and the nut driver. Now you can ignite and operate the furnace. Follow the ignition procedure specific to your furnace model.