A Pressure Switch Furnace Error

Testing

• The pressure switch is a round switch on the outside of your furnace near the draft inducer motor. Use an ohmmeter to test the switch after first turning off the power to your furnace. Begin your test by touching the tips of the meter’s probes together to zero them out. Set the meter to 24 volts and attach the black probe to a metal part of the furnace to ground it. Place the tip of the red probe on the metal tube that connects the pressure switch to the draft inducer motor. Check the reading. You should see a reading of between 24 and 28 volts. A reading less than 24 volts means the switch isn’t working and needs to be replaced.

Blower

• If your pressure switch is all right, you might have a problem with your inducer blower. Faulty inducer blowers are a common reason why pressure switches don’t close. When the thermostat calls for heat, the inducer starts creating a draft by blowing air. When there’s a good draft, the pressure switch is supposed to close and the control board fires up the igniter and opens the gas valve. However, the pressure switch won’t close if the inducer blower isn’t working correctly.

Restrictions

• A restricted vent can keep your pressure switch from closing. Inspect the tube that runs from your pressure switch to the draft inducer for condensation. Use a coat hanger to clear a clogged inducer nipple. A blockage in the water drain line could cause problems in the pressure switch tubing, if you have a high-efficiency condensing furnace. Use a wet vacuum to open the drain.

Other Problems

• If you have a pressure switch error message and you can hear gas hissing, you might have a broken igniter or the relay that sends power to the igniter is broken. The hissing sound should stop when the flame sensor fails to detect a flame, and your furnace shuts down. Carbon deposits on the surface of the heat exchanger can also keep pressure switches open as can high winds.