Carrier Furnace Pressure Switch Open – Meaning, Causes & Fix

CarrierFurnaceUpdated 2026-10-01

A "Pressure Switch Open" error on a Carrier furnace is a safety-related fault indicating that the furnace's pressure switch — a critical safety device that verifies the inducer draft motor is exhausting combustion gases properly — is not closing when the furnace's control board expects it to. The pressure switch is a normally-open switch that closes when the inducer motor creates sufficient negative pressure (draft) in the heat exchanger, confirming that combustion gases will be safely exhausted before the gas valve opens. When the switch stays open, the furnace's control board halts the heating cycle and displays a pressure switch error code (often a 3-blink or 31 code on Carrier furnaces). This guide explains what triggers a pressure switch open error and how to resolve it. For related HVAC errors, see our Goodman Furnace Open Limit Switch guide and our Carrier Thermostat Offline guide.

Quick Answer

What it means

The furnace's pressure switch — a safety device that verifies the inducer draft motor is exhausting combustion gases — is not closing when the control board expects it to, causing the furnace to halt the heating cycle.

Severity

High

Can you fix it yourself?

Partially — basic checks (venting, intake blockage, drain cleaning) are DIY, but a failed inducer motor, a bad switch, or a cracked heat exchanger requires a licensed HVAC technician.

Call a technician when

Call a licensed HVAC technician if the pressure switch open error persists after clearing vent and intake blockages and cleaning the condensate drain, as this indicates a failed inducer, a bad switch, or a heat exchanger issue.

What Does Pressure Switch Open Mean?

Carrier furnaces (and most modern condensing furnaces) use a pressure switch as a critical safety interlock. The switch is a normally-open device mounted on the inducer housing or connected to the heat exchanger via a rubber pressure-sensing hose. The furnace's ignition sequence requires the inducer motor to start and create a minimum negative pressure (draft) in the heat exchanger before the gas valve is allowed to open. The pressure switch senses this draft and closes when it reaches the set threshold.

If the control board does not detect the pressure switch closing within a few seconds of the inducer motor starting, it aborts the ignition sequence, locks out the gas valve, and displays a pressure switch error. The furnace will typically retry a few times before locking out entirely and requiring a power cycle to reset.

Common causes include:

1. Blocked exhaust vent: A blockage in the exhaust vent pipe (bird nest, debris, ice, snow) prevents the inducer from creating the required draft. 2. Blocked combustion air intake: On two-pipe (direct vent) furnaces, a blockage in the intake pipe prevents combustion air from entering, reducing draft. 3. Failed or weak inducer motor: The inducer motor is failing to spin at full speed or has failed entirely, so it can't create enough draft. 4. Cracked or loose pressure switch hose: The rubber hose connecting the pressure switch to the heat exchanger is cracked, loose, or kinked, so the switch doesn't sense the actual draft. 5. Faulty pressure switch: The switch itself has failed and won't close even with proper draft. 6. Plugged condensate drain: A blocked condensate drain can back up water into the pressure switch hose or heat exchanger, reducing draft. 7. Cracked heat exchanger: A cracked heat exchanger causes abnormal pressure readings that prevent the switch from closing — a serious safety issue.

This pressure-safety fault is one of several furnace safety interlock errors, conceptually related to the Goodman open limit switch error which is another safety-shutdown condition.

What Causes This Error?

Blocked exhaust vent pipe — bird nest, debris, ice, or snow preventing the inducer from creating required draft
Blocked combustion air intake on two-pipe (direct vent) furnaces reducing available draft
Failed or weak inducer motor not spinning at full speed or failing entirely
Cracked, loose, or kinked pressure switch sensing hose preventing the switch from sensing actual draft
Faulty pressure switch that won't close even with proper draft present
Plugged condensate drain backing water into the pressure switch hose or heat exchanger, reducing draft
Cracked heat exchanger causing abnormal pressure readings that prevent the switch from closing — a serious safety issue
Incorrectly sized or aftermarket pressure switch installed with the wrong pressure rating for the furnace model

How to Fix Pressure Switch Open

1
Turn off power to the furnace and check the exhaust vent. At the breaker or furnace disconnect, turn off power to the furnace. Outside, inspect the exhaust vent termination for bird nests, debris, ice, or snow and clear any blockage. This is the most common and easiest DIY cause to check.
2
Check the combustion air intake (on two-pipe furnaces). On direct-vent installations with a separate intake pipe, inspect the intake termination outside for blockage and clear it. A blocked intake reduces draft just as a blocked exhaust does.
3
Inspect the pressure switch hose. Remove the furnace's upper blower compartment door and locate the pressure switch (a small disc-shaped switch with a rubber hose attached). Inspect the hose for cracks, looseness, or kinks, reseat it firmly on both fittings, and replace the hose if it's cracked or brittle.
4
Clean the condensate drain. A plugged condensate drain can back water up into the pressure switch hose and reduce draft. Locate the condensate drain line (typically a PVC pipe exiting the furnace), flush it with water or a wet/dry vacuum to clear any blockage, and confirm water flows freely.
5
Restore power and observe the ignition sequence. Replace the furnace doors, restore power, set the thermostat to call for heat, and watch the furnace's ignition sequence through the blower door window. If the inducer motor starts and the pressure switch closes (audible click), the furnace should proceed to ignition — if it does, the issue is resolved.
6
Call a licensed HVAC technician if the error persists. If the pressure switch open error returns after clearing vent blockage, inspecting the hose, and cleaning the condensate drain, call a licensed HVAC technician. The cause is likely a failed inducer motor, a faulty pressure switch, or a cracked heat exchanger — all of which require professional diagnosis and parts replacement, and a cracked heat exchanger poses a carbon monoxide risk.

When to Call a Professional

If the pressure switch open error persists after clearing vent blockage, inspecting the pressure switch hose, and cleaning the condensate drain, call a licensed HVAC technician. A failed inducer motor ($300-$700 including parts and labor), a faulty pressure switch ($150-$300), or a cracked heat exchanger ($1,500-$3,500, or furnace replacement) all require professional diagnosis. A cracked heat exchanger is a serious carbon monoxide risk and requires immediate furnace shutdown — do not attempt to run the furnace if a cracked heat exchanger is suspected.

Can You Still Use the Device?

A Carrier furnace displaying a pressure switch open error will not heat — the control board locks out the gas valve as a safety measure, so the furnace will not produce heat until the pressure switch issue is resolved. The furnace's blower may continue to run (in a failed-ignition cool-down mode), but no heat will be produced. Pending heating calls will retry automatically a few times before the furnace locks out entirely, requiring a power cycle to reset.

Official Resources

Disclaimer: This article is for informational purposes only and does not constitute professional repair advice. CodeExplain is not affiliated with Carrier or any other brand mentioned. Always consult a qualified technician for complex repairs.