The pressure switch is a small safety device that keeps a gas furnace from lighting until it has proven the venting system is working. When it fails to close, the furnace stops partway through startup: the inducer fan runs, but the igniter never glows and the burners never light. It’s behind some of the most common furnace error codes, and the cause is usually something blocking the vent or the switch’s hose, not the switch itself.
Last updated: November 2026. Applies to most induced-draft gas furnaces (80% and 90%+ efficiency).
What the pressure switch does
Modern gas furnaces use an inducer motor, a small fan that pulls combustion gases through the heat exchanger and pushes them out the flue. Before the furnace lights, the control board needs proof that the inducer is actually moving air. That’s the pressure switch’s job.
The switch is connected to the inducer housing by a thin rubber hose. When the inducer starts, it creates suction. The suction pulls a diaphragm inside the switch, and the switch closes, telling the control board it’s safe to continue. If the suction isn’t strong enough, the switch stays open and the furnace won’t light.
The startup sequence looks like this:
| Step | What happens | If the pressure switch fails |
|---|---|---|
| 1 | Thermostat calls for heat | — |
| 2 | Control board checks that the pressure switch is open | Switch stuck closed: furnace stops before the inducer starts |
| 3 | Inducer motor starts | — |
| 4 | Pressure switch should close within a few seconds | Switch stays open: inducer runs, nothing else happens |
| 5 | Igniter heats, gas valve opens, burners light | — |
| 6 | Blower starts and warm air comes out | Switch opens while running: burners shut off |
Pressure switch error codes by brand
Most furnaces report pressure switch problems with a blinking light on the control board:
| Brand | Switch won’t close (open) | Switch stuck closed |
|---|---|---|
| Carrier, Bryant, Payne | Code 31 | Code 23 |
| Goodman, Amana | 3 flashes | 2 flashes |
| Trane, American Standard | 3 flashes | 3 flashes (one code covers both) |
| Lennox | E223, E225, E227 | E224, E226 |
Codes vary by model and age, so always check the label on your furnace door. See our guides to Carrier, Goodman, Trane and Lennox codes, or furnace blinking light codes for other brands.
Why a pressure switch won’t close
The switch almost always fails to close because it isn’t getting enough suction, not because it’s broken. Common causes, from most to least likely:
- Blocked vent or intake pipe. On high-efficiency furnaces, the white PVC pipes outside can be blocked by snow, ice, leaves or a bird’s nest. On older furnaces, a blocked chimney or flue does the same.
- Clogged condensate drain (90%+ furnaces). When water can’t drain from the inducer or collector box, it backs up and blocks the airflow the switch senses. A gurgling sound at startup is a classic sign.
- Cracked, kinked or water-filled hose. The thin hose between the switch and the inducer can crack with age or hold a drop of water that blocks it.
- Clogged switch port. The small tap on the inducer housing where the hose connects can fill with rust or debris.
- Weak or failing inducer motor. If the inducer spins slowly or is noisy, it may not create enough suction.
- Failed pressure switch. Less common than the causes above, but switches do wear out.
Why a pressure switch is stuck closed
If the control board sees the switch closed before the inducer starts, it assumes something is wrong and won’t continue. This is usually a failed switch with its contacts stuck, or a wiring problem. Less often, it’s a control board fault.
What you can check safely
Turn the furnace off at the thermostat before checking anything near the unit, and don’t remove panels that require tools unless you’re comfortable doing so.
- Check the vent pipes outside. Clear any snow, ice, leaves or debris from the intake and exhaust pipes. This alone fixes many winter pressure switch errors.
- Check the condensate drain. On a 90%+ furnace, look for water in the drain trap or the hoses, and make sure the drain line isn’t clogged or frozen. See AC condensate line clogged, since the same drain often serves both.
- Inspect the hose. With power off, look at the small hose from the switch to the inducer. Replace it if it’s cracked or brittle, and blow gently through it to clear water. Never blow into the switch itself.
- Listen to the inducer. A loud, grinding or slow inducer may be failing.
- Reset the power. After clearing a blockage, turn the furnace off at its service switch for 30 seconds and back on to clear a lockout.
What not to do
Never bypass or jumper a pressure switch, even briefly to “test” the furnace. The switch exists to stop the furnace from lighting when combustion gases can’t vent safely. Running a furnace with a blocked vent can push carbon monoxide into your home. Make sure you have working carbon monoxide detectors on every level of the house.
When to call an HVAC technician
Call a licensed technician if the vents and drain are clear but the error keeps coming back, the inducer motor is noisy or slow, or the switch needs testing or replacement. A technician will measure the actual suction with a manometer and compare it to the rating printed on the switch, which tells them whether the switch, the inducer or the venting is at fault.
Typical costs: a replacement pressure switch usually costs $150 to $400 installed, and an inducer motor replacement often $400 to $1,000, depending on the furnace and region. For more, see furnace not turning on.
If the inducer itself won’t start or is noisy, see furnace inducer motor problems.
Frequently asked questions
What are the signs of a bad furnace pressure switch?
The inducer fan runs but the furnace never lights, the furnace shows a pressure switch error code, or it starts and shuts off after a few seconds. Remember that the cause is usually a blockage, not the switch itself.
Can a dirty filter cause a pressure switch error?
Not directly. A dirty filter usually trips the high limit switch instead. See furnace high limit switch.
Why does my furnace pressure switch error only happen in very cold weather?
Snow or ice can block the outdoor vent pipes, and condensate can freeze in the drain or the pipes. Check outside after storms and very cold nights.
Can I replace a pressure switch myself?
Some homeowners do, but the replacement must match the original’s pressure rating exactly. A wrong switch can let the furnace run unsafely. If you’re not sure, have a technician do it.
Disclaimer: this guide is for general information only. Gas furnaces involve gas, combustion and high voltage. If you smell gas, leave the house and call your gas utility from outside. When in doubt, contact a licensed HVAC professional.