Furnace Pressure Switch Not Working: Causes, Diagnosis, and Fixes

When A Furnace Pressure Switch Not Working Prevents Heating, Homeowners Face A Common But Fixable Problem. This Guide Explains Causes, Safety Steps, Diagnostic Tests, Repair Options, And Cost Expectations To Help Restore Reliable Furnace Operation.

Issue	Likely Cause	Typical Action
Pressure Switch Won’t Close	Blocked Vent/Inducer Or Faulty Switch	Clear Vent, Test Switch, Replace If Needed
Intermittent Operation	Loose Wiring Or Weak Switch	Tighten Connections, Replace Switch
False Open Reading	Vacuum Hose Leak Or Clogged Drain	Inspect Hose, Clean Drain, Replace Hose
No Inducer Motor Operation	Blown Fuse, Control Board Or Motor Failure	Check Power, Test Inducer, Replace Component

How A Furnace Pressure Switch Works

The furnace pressure switch Is A Safety Device That Verifies The Inducer Motor And Venting System Create Proper Airflow Before Ignition. It Senses Negative Pressure Via A Small Vacuum Hose And Sends A Signal To The Control Board To Allow The Ignition Sequence.

Key Functions: Prevents Gas Flow Without Proper Venting, Ensures Combustion Airflow, And Protects Against Carbon Monoxide Hazards.

Common Causes Of A Pressure Switch Failure

Blocked Venting Or Flue

Obstructions In The Vent Pipe Or Chimney, Such As Bird Nests, Debris, Or Snow, Can Prevent Proper Airflow, Causing The Inducer To Create Insufficient Negative Pressure And Keep The Switch Open.

Clogged Condensate Drain Or Trap

Condensing Furnaces Produce Condensate That Must Drain Properly. A Clogged Trap Or Drain Can Cause Backpressure Or Moisture Buildup That Interferes With The Pressure Sensing System.

Damaged Or Disconnected Vacuum Hose

The Small Rubber Hose Connecting The Inducer Housing To The Pressure Switch Can Crack, Become Loose, Or Get Blocked By Debris, Leading To False Open Or Closed Readings.

Faulty Pressure Switch

Aging Or Defective Switch Contacts Or Diaphragm Fail To Move Under Proper Vacuum, Causing The Control Board To Think Venting Is Unsafe Even When It Is Functional.

Inducer Motor Problems

If The Inducer Motor Fails To Start Or Runs Weakly Due To A Motor Fault Or Electrical Issue, The Required Vacuum Won’t Reach The Switch.

Control Board Or Electrical Issues

Loose Wiring, Blown Fuses, Or A Faulty Control Board Can Prevent Power From Reaching The Inducer Or Reading The Switch Signal.

Troubleshooting Steps For A Furnace Pressure Switch Not Working

Safety First

Turn Off Power To The Furnace At The Breaker And Shut Off Gas If Any Component Needs Disassembly. Work With Caution Around Gas And Electrical Systems.

Visual Inspection

Check The Vent Terminal For Obstructions, Inspect The Vacuum Hose For Cracks Or Disconnections, And Look For Signs Of Water Or Debris Around The Inducer And Switch.

Listen And Observe

Restore Power And Put The Thermostat Call For Heat. Listen For The Inducer Motor Starting. If The Inducer Starts But The Furnace Shuts Down Before Ignition, The Pressure Switch May Not Close.

Test The Vacuum Hose

Disconnect The Vacuum Hose At The Switch And Blow Gently To Check For Blockage. Replace Any Brittle Or Collapsed Hose. Use A Small Wire Or Compressed Air To Clear Debris From The Inducer Port If Accessible.

Measure Switch Continuity

With Power Off, Use A Multimeter To Check Continuity Across The Switch Contacts. The Switch Should Be Open At Rest And Close When Inducer Vacuum Is Applied.

Apply Vacuum Manually

Using A Hand Vacuum Pump, Apply Negative Pressure To The Switch Port While Measuring Continuity. If The Switch Closes Under Applied Vacuum, The Inducer Or Venting Is Likely The Problem.

Check Inducer Motor Current

Measure The Inducer Motor’s Running Amps And Compare To The Motor’s Nameplate. Low Amps May Indicate Mechanical Binding Or Electrical Weakness.

How To Test And Replace A Pressure Switch

Testing Procedure

1. Turn Off Power To The Furnace.
2. Locate The Pressure Switch On The Furnace Door Or Cabinet.
3. Disconnect The Vacuum Hose From The Switch And Inspect It.
4. Use A Multimeter Set To Ohms To Measure Across The Switch Terminals.
5. Power On Furnace And Start A Heat Call; Observe If The Switch Closes When Inducer Runs.
6. Alternatively, Use A Hand Vacuum Pump To Simulate Inducer Vacuum And Watch For Continuity Change.

Replacement Steps

1. Turn Off Power And Gas To The Furnace.
2. Note Wiring Location Or Take A Photo For Reconnection.
3. Remove The Two Wires And The Hose From The Old Switch.
4. Unscrew Or Unclip The Switch From The Furnace Mount.
5. Install The New Switch, Reattach Hose And Wiring, Then Restore Power.
6. Test Operation By Running A Heat Call And Confirming The Furnace Proceeds To Ignition.

Tip: Replace The Vacuum Hose Along With The Switch If It Shows Any Wear Or Hardening.

When To Call A Professional

Call A Licensed HVAC Technician If The Troubleshooting Steps Don’t Identify The Problem Or If The Inducer Motor, Control Board, Or Gas Valve Requires Service. Professionals Have Specialized Tools For Diagnosing Flue Obstructions, Motor Electrical Tests, And Combustion Safety Checks.

Immediate Service Recommended If There Is A Strong Gas Smell, Soot, Continuous Cycling, Or Any Carbon Monoxide Alarm Activation.

Costs And Parts To Expect

A Typical Pressure Switch Part Costs $20 To $120 Depending On Furnace Make And Model. Total Replacement Labor Usually Adds $100 To $300 For A Standard Service Call, But Costs Vary Regionally.

Item	Estimated Cost
Pressure Switch (Part)	$20–$120
Vacuum Hose	$5–$20
Inducer Motor	$150–$450
Service Call + Labor	$100–$300+

Choosing A Professional Warranty Or Club Service Plan Can Provide Reduced Repair Costs For Future Failures.

Preventive Maintenance To Avoid Pressure Switch Problems

• Schedule Annual Furnace Tune-Ups To Inspect Inducer, Pressure Switch, And Venting.
• Keep Outdoor Vent Terminals Clear Of Debris, Snow, And Ice.
• Replace Air Filters Regularly To Reduce System Strain.
• Clean Condensate Traps And Drains On Condensing Furnaces To Prevent Blockage.
• Inspect Vacuum Hoses For Wear During Maintenance.

Safety Considerations And Carbon Monoxide Risk

Because The Pressure Switch Directly Prevents Unsafe Ignition, Any Malfunction Raises The Risk Of Incomplete Combustion Or Carbon Monoxide Entry If Other Safeties Fail. A Properly Working Switch Reduces These Risks By Ensuring Correct Venting Before Gas Ignites.

Install At Least One Plug-In Or Hardwired Carbon Monoxide Alarm Near Sleeping Areas And Test Alarms Regularly. If The Furnace Smells Of Gas Or A CO Alarm Activates, Evacuate And Call Emergency Services.

Frequently Asked Questions

How Long Does A Pressure Switch Last?

Pressure Switches Can Last Many Years But May Fail Sooner Due To Moisture, Corrosion, Or Mechanical Wear. The Typical Lifespan Is Often Similar To Other Furnace Controls, Ranging From 5 To 20 Years Depending On Conditions.

Can A Dirty Filter Cause The Pressure Switch To Fail?

A Dirty Filter Raises System Restriction And Can Cause The Furnace To Overwork, Making The Inducer Less Effective. While A Filter Alone Rarely Damages The Switch, It Contributes To Conditions That Prevent Proper Vacuum.

Is It Safe To Jumper The Pressure Switch?

Temporarily Jumpers For Troubleshooting Should Only Be Done By Qualified Technicians; Bypassing The Switch Eliminates A Critical Safety Interlock And Can Create Dangerous Conditions If Ignition Occurs Without Proper Venting.

What If The Switch Is Intermittent?

Intermittent Switch Behavior Often Points To Loose Wiring, A Weak Switch, Or A Partially Blocked Vent/Hose. Replacing The Hose And Switch Is A Common Fix, But A Technician Should Address Recurring Issues.

Resources And Further Reading

Manufacturers’ Service Manuals, HVAC Training Guides, And Local Utility Safety Pages Provide Detailed Procedures For Specific Furnace Models. For Complex Diagnostics, Consult A Licensed HVAC Professional Or Refer To The Furnace Manufacturer’s Troubleshooting Flowchart.

U.S. Consumer Product Safety Commission And EPA Offer Safety Resources Related To Indoor Air Quality And Carbon Monoxide.