Pressure Switch Failures: Why Your Furnace Keeps Locking Out
Your furnace has a built-in safety device called a pressure switch that verifies the inducer motor is pulling combustion gases out of the heat exchanger before the burners light. If the pressure switch doesn’t close, the furnace locks out and won’t fire — even though everything else seems fine.
This is one of the most frustrating furnace problems because the system looks like it’s trying to start. You hear the inducer motor kick on, wait a few seconds, and then… nothing. The burners never light.
How the Pressure Switch Works
The inducer motor creates a draft through the heat exchanger. That draft pulls on a small rubber hose connected to the pressure switch. When the draft is strong enough, the switch closes and tells the control board it’s safe to open the gas valve and light the burners.
If the switch doesn’t close — or closes and then opens during operation — the board shuts everything down as a safety precaution. No draft means combustion gases could back up into your home.
Common Causes of Pressure Switch Failures
Clogged drain line or condensate trap. On high-efficiency furnaces (90%+), condensation drains through a trap. If the trap is blocked, water backs up into the inducer housing and prevents proper draft. Cleaning the trap often fixes the problem without replacing any parts.
Cracked or disconnected hose. The small rubber hose between the inducer and the pressure switch can crack, slip off, or fill with condensation. Check it for damage and make sure it’s firmly connected at both ends.
Weak inducer motor. If the inducer motor is getting old, it may not spin fast enough to create adequate draft. The motor runs, but the pressure switch never sees enough negative pressure to close.
Bad pressure switch. The switch itself has a rubber diaphragm inside that can wear out or stick. If you’ve ruled out the other causes, the switch may need replacing.
Testing a Pressure Switch
With the inducer running, use a multimeter to check for continuity across the switch terminals. If the inducer is running strong and you’re reading open (no continuity), the switch is likely bad. You can also gently suck on the hose port — if the switch doesn’t click closed, it’s stuck.
Open To Public HVAC Parts stocks pressure switches for Goodman, Rheem, Carrier, Trane, Lennox, and universal replacements. Bring your old switch in and we’ll match it by the pressure rating stamped on the housing. Free testing at the counter. 510+ Google reviews. Since 1998.
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