How to Reset a Furnace Pressure Switch

Furnace pressure switches are safety devices that shut your system down when something blocks the intake or exhaust vents. When one trips, your furnace simply stops running—which feels broken, but it's actually working correctly. The switch isn't stuck; it's protecting your home from carbon monoxide or flame rollout. Resetting it means finding and fixing what tripped it in the first place, not just flipping a lever. Most resets take less than an hour and require nothing more than a ladder and your hands.

1. Kill the Power First. Locate the furnace power switch—usually a toggle switch on the side of the unit or on the wall near it. Flip it to OFF. Wait 30 seconds for the system to fully power down. This clears any residual charge and gives you a safe window to work.
2. Find the Intake Vent. The intake vent draws air from outside into the combustion chamber. Look for the white or gray PVC pipe running from the furnace up through the wall or roof. Check both ends—the outdoor opening and where it enters the furnace. Remove any visible debris: bird nests, leaves, snow, ice, or lint buildup. If the vent is iced over, pour warm (not boiling) water on the exterior opening to melt it, or wait for warmer weather.
3. Clear the Exhaust Path. The exhaust vent expels combustion gases outside. It's typically another PVC pipe near the intake. Follow it from the furnace to the exterior opening. Look for lint, dust, insect nests, or condensation ice blocking the outlet. Clear any obstruction by hand or with a soft brush. If condensation has frozen inside, apply heat carefully with warm water or a heat gun from outside—never use an open flame.
4. Verify Hose Connections. Locate the pressure switch—a round or rectangular device with two hoses running from it. These hoses connect to the intake and exhaust pipes and sense whether air is moving through them. Check that both hoses are firmly attached and not cracked or kinked. If a hose has disconnected, reconnect it by pushing it onto the barbed fitting until it clicks. If a hose is cracked or very loose, note it for replacement—but clearing the vents usually solves the problem first.
5. Power Back On. Turn the power switch back ON. The furnace should power up within a few seconds. Listen for the blower motor to engage—you'll hear it spin up and feel air moving through the vents. If you hear a soft click from the pressure switch, that's normal; it's confirming airflow. The system should run normally now. If it shuts off again within a minute, the switch is still detecting a problem; check the vents again.
6. Confirm Heat Flows. Set your thermostat to HEAT and raise the temperature above the current room temperature. The furnace should ignite and the blower should run continuously until the house reaches that temperature. Feel warm air coming from your floor or wall registers. Once the house warms, the furnace should cycle off and back on as needed to maintain temperature. If it's running smoothly, you're done.
7. Stay Ahead Seasonally. Mark your calendar to inspect the intake and exhaust vents before winter and after spring storms. Clear any debris you find. In fall, check for leaves; in spring, check for bird nests; in winter, clear any ice forming at the outdoor opening. A quick 5-minute visual check every season prevents most pressure switch trips.