This guide is written for techs to help troubleshoot pressure switches. I will explain what they are, how they fail, how to fix them, and when to look elsewhere.
If you need a complete overview of furnace parts, you can find that guide here.
Article Contents
The Basics
At its core, a pressure switch is just a mechanical safety switch. It works similarly to a snap disk, except instead of reacting to temperature, it reacts to suction. A spring holds the switch open until air pressure pulls on a diaphragm and closes the contacts. That pressure comes from the inducer motor, which creates suction at the start of a heating cycle. Once the diaphragm flexes and the contacts close, voltage is allowed to pass through.
Because they react to air pressure, pressure switches are used to prove draft. You’ll find them mounted to the inducer housing or the collector box, with a small hose running to a pressure port. They help the control board verify that the furnace is venting properly before allowing ignition to continue.
How the Board Uses the Pressure Switch
When a furnace gets a call for heat, the control board checks two things:
First, it confirms all limit and rollout switches are closed. Then, it checks the pressure switch circuit to make sure the switch is open. The pressure switch should only close once the inducer has started. If the switch is already closed, you’ll get a “pressure switch stuck closed” error.
Next, the inducer motor starts and a timer begins. Suction builds, the diaphragm flexes, and the switch closes. If the board doesn’t see this happen within a few seconds, it throws a “pressure switch open” fault.
When everything works properly, the switch closes just after the inducer starts, and the board moves on to ignite the burners.
Common Pressure Switch Problems
The most common pressure switch problem? The pressure switch is doing its job. It’s reading a fault elsewhere and shutting the system down.
Things that can prevent the switch from closing:
- Blocked intake or exhaust venting
- Water trapped in the inducer or collector box
- Clogged or kinked pressure tubing
- A cracked or disconnected hose
- Weak inducer motor not generating enough suction
- Clogged condensate drain or unprimed trap
- Wiring or board issues (voltage not sent or signal not received)
In rare cases, the switch itself can fail. It might stick open or closed, or its calibration might drift.
How to Test a Pressure Switch
Basic Function Test
Disconnect the wires from the switch, but leave the hose connected. Set your multimeter to resistance (Ω). With no suction, the switch should read OL. Apply light suction to the hose (typically done by sucking on the hose with your mouth like it was a straw). The reading should drop to near 0 Ω. If the switch doesn’t open and close with suction, it’s bad.
Testing Switch Pressure
Look at the rating on the switch. It’s usually rated in inches of water column (” WC). Tee a manometer into the hose between the inducer and switch. Start the furnace and watch the pressure. If suction reaches or exceeds the labeled spec but the switch stays open, the switch is faulty. If suction never reaches that value, the problem is upstream: venting, inducer, or drain.
Board Voltage Test
Leave the wires attached and set your meter to volts. Probe the terminals during a heat call. If you read 24–28V and the switch never closes, the switch may be stuck. If you read 0V and the switch is testing open on resistance, the board isn’t sending voltage and may be bad.
How to Bypass a Pressure Switch (for Testing Only)
If you’ve confirmed the switch is bad and want to continue diagnosing the system, you can temporarily bypass it. But timing matters.
Start by clipping a jumper to one terminal only. Call for heat. Once the inducer starts, connect the other side of the jumper to the second terminal within 2–3 seconds. That simulates the correct sequence. If the board sees this, it moves on.
Important: Only do this if you’re certain the pressure switch is the problem. Never leave a furnace running with the switch bypassed.
Pressure Switch Variations
- Two-stage systems have two switches: one for low, one for high fire
- Modulating systems may have three
- Differential (dual-hose) switches measure pressure difference between two points, often the inducer and collector box
- Some single-stage units use two single-port switches instead of one dual
Always match replacement pressure switches with exactly the same pressure rating, port layout, and voltage handling. Substituting the wrong one will cause more problems.
Other Things to Check
- Make sure tubing runs downhill back to the collector or inducer
- Check for water or debris inside the tubing
- Look for sags or kinks that could trap water
- Confirm proper slope and support of vent piping
- Make sure the condensate trap is primed and draining
When you’re chasing a pressure switch fault, start at the switch, but know that often times the switch is doing it’s job and alerting you to a problem elsewhere.
