Furnace Pressure Switch Open Fault: Causes, Diagnosis, and Fixes

A pressure switch open fault on a furnace signals that the system did not detect the expected airflow or vent pressure needed for safe burner operation. This article explains common causes, diagnostic steps, DIY checks, likely repairs, safety considerations, and when to call a qualified technician. The goal is to help homeowners understand the issue clearly and make informed decisions about fixes and maintenance.

Symptom	Likely Cause	Typical Fix
Pressure switch open fault	Blocked vent/inducer, clogged condensate trap, damaged hose, faulty switch	Clear vent, clean trap, replace hose/switch, repair inducer
Intermittent fault	Loose wiring, intermittent inducer operation, weak switch spring	Tighten connections, test inducer motor, replace switch
Noisy inducer	Worn motor bearings or debris	Replace inducer motor or clean debris

What The Pressure Switch Does

The pressure switch is a safety device that verifies the furnace inducer motor is creating the correct draft before allowing gas valves to open. It measures negative or positive air pressure in the vent/combustion chamber using a small vacuum hose and an internal diaphragm or reed switch. If pressure readings are out of range, the switch remains open to prevent ignition.

Why A Pressure Switch Open Fault Occurs

Several mechanical and electrical issues can cause the pressure switch to stay open. Common categories include airflow and venting problems, condensate or blockage in the switch tubing, failed pressure switch components, inducer motor faults, and control or wiring issues. External conditions like severe cold or ice can also affect vent pressure.

Airflow And Venting Problems

Obstructed exhaust or intake vents, animal nests, blocked chimneys, snow, and collapsed vent pipes can prevent the inducer from establishing the necessary draft. Any restriction in the vent system can trigger a pressure switch open fault.

Condensate And Blockage In Hose

High-efficiency furnaces produce condensate that drains through traps and tubing. A clogged condensate trap or tubing can prevent the pressure switch from sensing the proper pressure. Debris or water in the switch hose can block the vacuum path.

Faulty Pressure Switch Or Hose

The switch itself can fail due to worn diaphragms, corroded contacts, or a broken internal spring. The small vacuum hose connecting the switch to the combustion chamber may crack, split, pinch, or become disconnected, causing a false open condition.

Inducer Motor Or Fan Problems

If the inducer motor fails, runs slowly, or has intermittent operation, it will not generate enough draft pressure. Bearings can wear, vents can load with soot, or the motor capacitor (if present) may fail, all producing insufficient pressure.

Control Board And Wiring Failures

Loose wiring, bad connectors, or a failing control board can prevent a closed signal from the pressure switch from being recognized. Conversely, incorrect voltage or damaged wiring between the switch and board can create an open-fault symptom.

Interpreting Error Codes And LED Blink Patterns

Most modern furnaces use LED blink codes to communicate faults. The control board manual or label inside the furnace cabinet describes the blink pattern for “pressure switch open” or related pressure faults. Document the blink pattern before troubleshooting and consult the furnace model guide to map the code to the specific fault.

Safety Precautions Before Any Inspection

Turn off power to the furnace at the service switch and the breaker before opening the access panel. Shut off gas supply if performing work involving burners or gas valve components. Wear safety glasses and gloves, and avoid interfering with gas components unless qualified. If there is any gas odor, evacuate and call the gas company or emergency services.

Step-By-Step Diagnostic Checklist

The following checklist guides a logical diagnosis from simple to complex causes. Perform these steps in order to reduce unnecessary parts replacement and to isolate the root cause.

1. Record the error code blink pattern and reset the furnace to observe behavior.
2. Visually inspect the vent termination and intake for obstructions (leaves, nests, snow).
3. Inspect the condensate trap and drain for clogs; clear and flush if needed.
4. Examine the pressure switch vacuum hose for cracks, kinks, or disconnection.
5. Listen for inducer motor operation during startup; note unusual noises or failure to run.
6. Check wiring at the pressure switch and control board for loose or corroded connections.
7. Use a multimeter to test switch continuity when the inducer is operating (see testing section).
8. If the switch closes but the board still shows open, investigate control board faults.

How To Test The Pressure Switch Safely

Testing requires a multimeter and basic comfort with electrical diagnostics. Only perform if comfortable and after power is off while connecting test leads. Reapply power only when testing during inducer operation and follow safety steps.

Continuity Test With Power Off

Disconnect power. Remove the wires from the pressure switch terminals. Set the multimeter to continuity or ohms. With the switch at rest (no inducer), many switches show open or closed depending on design. Refer to the furnace manual for expected at-rest state. A failed switch often reads infinite resistance in both states.

Live Test During Inducer Operation

Reinstall wiring if needed and restore power. Start a heating call so the inducer runs. Carefully measure voltage across the switch or test continuity: when inducer reaches operating speed and correct pressure is present, the switch should close and show continuity or 0 volts across the control terminals depending on configuration. If the switch never closes while inducer runs, the switch, inducer, or venting is likely at fault.

DIY Fixes Homeowners Can Safely Perform

Several non-electrical fixes can resolve a pressure switch open fault. Homeowners should focus on vent clearing, condensate and hose cleaning, and basic wiring checks.

• Clear external vent and intake terminations of debris, snow, or bird nests.
• Inspect and replace a brittle or collapsed vacuum hose; these are inexpensive and replaceable by most homeowners.
• Clean or replace the condensate trap and ensure the drain is flowing.
• Tighten loose wire terminals and clean corroded contacts on the switch and control board.

Repairs That Typically Require A Technician

Replacing the pressure switch, inducer motor, control board repairs, advanced electrical diagnosis, and gas-related repairs should be handled by a licensed HVAC technician. These tasks require specialty tools, safety training, and in some jurisdictions, licensed work for gas appliance repairs.

Typical Parts, Cost Ranges, And Time Estimates

Costs vary by furnace model and region. These are ballpark estimates to aid planning.

Repair	Parts Cost	Labor Time
Replace vacuum hose	$5–$30	10–30 minutes
Replace pressure switch	$30–$150	30–60 minutes
Clean condensate trap/drain	$0–$50	20–45 minutes
Replace inducer motor	$150–$500+	1–3 hours
Control board service/replace	$200–$800	1–3 hours

Preventive Maintenance To Avoid Pressure Switch Faults

Regular maintenance reduces the chance of pressure switch faults. Recommended tasks include annual professional tuneups, cleaning vents, replacing air filters, checking condensate drains each season, and inspecting the outdoor vent cap for nests or ice buildup. Routine checks keep venting and inducer performance within design limits.

When To Call A Professional

If basic checks (clearing vents, replacing hose, cleaning trap) do not clear the fault, or if the inducer motor fails to run, emits unusual noises, or there are wiring/control issues, call a licensed HVAC technician. Also call a pro immediately for any gas smell or suspected gas leak.

Common Misdiagnoses To Avoid

Replacing the pressure switch without checking venting or inducer function is a common wasteful repair. Similarly, ignoring the control board or wiring can lead to repeated faults after part replacement. A systematic approach avoids unnecessary parts costs.

Key Takeaways For Homeowners

A pressure switch open fault is a safety shutdown often caused by vent obstruction, condensate blockage, a bad vacuum hose, inducer failure, or switch failure. Start with safe visual checks and hose replacement, then test switch operation. If problems persist or if gas-related or electrical repairs are needed, contact a qualified technician. Document error codes and steps taken to help the technician diagnose and resolve the issue faster.