Things You'll Need

Instructions

1

Turn on the furnace. If it turns on normally, the pressure switch is open initially as it should be. If the furnace refuse to kick on, the pressure switch may be closed in error and this prevents the furnace from starting properly.
2

Listen to the furnace as it runs if it starts normally. Listen for the inducer blower to start, which is the fan that helps circulate the heat throughout the building. If the furnace suddenly shuts down when the inducer blower starts working, the pressure switch is a likely culprit since it incorrectly closes in response to the pressure change the blower causes.
3

Pull off the tubing that plugs into the pressure switch and inspect it for clogs. Blow into it to see if air travels through the tubing without difficulty. Unclogging the tube may correct problems with the pressure switch. Use the end of a wire coat hanger to fish out any clogs in the line before plugging it back into the pressure switch.
4

Place one lead of a voltmeter against a wire going into the pressure switch. Ground the other lead by holding it against the metal frame of the furnace. The switch should register 24 to 28 alternating current volts on the voltmeter if it is working properly. If you read less than that, the pressure switch likely needs replacement.