Lennox Furnace Pressure Switch Open With Inducer On: Causes and Fixes

The Lennox furnace showing the inducer motor running while the pressure switch reads open is a common fault that keeps the system from igniting. This article outlines probable causes, step-by-step troubleshooting, safety precautions, diagnostic tests with expected readings, parts to inspect or replace, estimated repair costs, and when to call a professional. The goal is to help homeowners or technicians isolate and resolve the issue efficiently while avoiding unsafe procedures.

Symptom	Possible Cause	Quick Action
Inducer Runs But Pressure Switch Open	Blocked vent/condensate, cracked hose, stuck switch, faulty inducer, electrical issue	Inspect vent and hose, check continuity, measure vacuum

How The Pressure Switch And Inducer Work

The inducer motor creates negative pressure in the combustion chamber and vent system to ensure safe exhaust flow before ignition. The pressure switch senses that negative pressure and closes its internal contacts to signal the control board that conditions are right for ignition. If the inducer runs but the pressure switch remains open, the board will prevent the furnace from proceeding to ignition for safety.

Common Causes Of Pressure Switch Open With Inducer On

Several issues can cause a pressure switch to remain open despite the inducer running. Common causes include vent blockages, clogged condensate traps or drains, disconnected or cracked pressure tubing, defective pressure switches, weak inducer motors that cannot pull enough vacuum, and control board or wiring faults.

Blocked or Restricted Venting

Bird nests, debris, ice, or snow at the termination cap can restrict airflow. Even partial restrictions reduce vacuum and prevent the pressure switch from closing.

Clogged Condensate Or Drain Issues

High-efficiency Lennox furnaces produce condensate that drains away. A clogged trap or drain can trap water and block the pressure sensing pathway, causing the switch to remain open.

Damaged Or Disconnected Pressure Tubing

Rubber or vinyl tubing runs from the inducer housing to the pressure switch. Cracks, kinks, or loose fittings let in air and prevent the necessary vacuum from reaching the switch.

Defective Pressure Switch

Mechanical switches can fail to operate even when proper vacuum is present. Contacts may stick or the diaphragm can rupture. Testing and replacement are often required to confirm failure.

Weak Inducer Motor Or Obstructed Blower Wheel

Inducers that run but produce insufficient vacuum due to worn bearings, electrical issues, or buildup on the blower wheel will not create enough pressure differential to close the switch.

Control Board Or Wiring Errors

Bad wiring, loose connectors, or a faulty control board input can falsely read the switch as open. Always verify wiring and voltages before replacing components.

Safety First: Precautions Before Troubleshooting

Always turn power off at the furnace switch or breaker before opening panels. Use appropriate personal protective equipment. If the furnace is gas-fired, shut off the gas supply before performing invasive repairs. Do not bypass the pressure switch or other safety devices to force the furnace to run.

Tools And Equipment Needed

• Multimeter with continuity and voltage functions
• Manometer or digital vacuum gauge (recommended) to measure inches of water column (in. WC)
• Screwdrivers, nut drivers, pliers
• Shop vacuum or compressed air for vent cleaning
• Replacement tubing and pressure switch (if needed)
• Flashlight and mirror for visual inspection

Step-By-Step Troubleshooting Procedure

This sequence helps isolate the cause efficiently. Each step includes expected results and follow-up actions.

1. Visual Inspection

Inspect the furnace vent termination, inducer housing, and pressure tubing. Look for blockages, animals, collapsed hose, or standing water in the condensate trap.If any obvious issues appear, clear the blockage and retest.

2. Check Pressure Tubing

With power off, disconnect the tubing at the pressure switch and inspect for moisture, cracks, or debris. Replace brittle or damaged tubing. Reconnect securely and retest operation.

3. Measure Vacuum With Inducer Running

Restore power and allow the inducer to run. Use a manometer to measure vacuum at the pressure switch tap; typical values for Lennox systems range from 0.5 to 2.0 in. WC depending on model. If vacuum is below the pressure switch setpoint, investigate venting or inducer performance.

4. Verify Pressure Switch Operation

With the inducer running, use a multimeter on continuity or voltage across the pressure switch terminals. The switch should show continuity (closed) when sufficient vacuum exists. If vacuum is present but the switch does not close, replace the switch.

5. Inspect Inducer Motor Performance

Measure inducer motor voltage and listen for unusual noises. Low voltage, slow spin, or wobble suggests motor or capacitor issues. A weak inducer may require motor replacement or cleaning of the blower wheel.

6. Check Condensate Trap And Drain

Remove and clean the condensate trap and verify the condensate drain is clear. Ensure proper trap placement and slope. Reinstall and retest.

7. Test Control Board Inputs And Wiring

Verify the wiring from the pressure switch to the control board is secure and free of corrosion. Check the control board for error codes on the LED or status indicators and follow Lennox diagnostic chart for the specific code.

Expected Readings And Values

Pressure switches commonly close around 0.5 to 1.5 in. WC. Manufacturer documentation or the switch label gives the setpoint. Inducer vacuum readings under load usually fall within that range. Multimeter continuity should show near-zero ohms when the switch is closed.

When To Replace Parts

Replace pressure tubing if cracked or clogged. Replace the pressure switch if it does not close when the correct vacuum is applied. Replace the inducer motor or blower wheel if the inducer cannot develop the required vacuum despite clear vents. Replace control board components only after verifying wiring and inputs.

Estimated Costs And Time

Parts and labor vary by region. Typical retail part costs: pressure switch $20–$80, pressure tubing $5–$15, inducer motor $150–$400, control board $200–$600. DIY repair of tubing or switch replacement can take 30–90 minutes; inducer motor or board replacement may take 1–3 hours. Professional service call adds diagnostic fees and labor.

Common Lennox Models And Pressure Switch Locations

Lennox high-efficiency models use a similar pressure switch layout near the inducer housing. The switch is usually a round or rectangular component mounted on the furnace cabinet with two hose connections or a single connection and two electrical terminals. Refer to the model’s schematic on the door panel for exact placement.

Diagnostic Flowchart Table

Step	Check	Result	Action
1	Visual Vent/Tubing	Blocked or damaged	Clear vent; replace tubing
2	Vacuum Measurement	Below setpoint	Inspect inducer and vent
3	Pressure Switch Continuity	No continuity despite vacuum	Replace pressure switch
4	Inducer Voltage/Spin	Low speed or noise	Repair/replace inducer motor
5	Control Board Input	Erratic or no signal	Check wiring; replace board if needed

Tips To Prevent Future Pressure Switch Issues

• Keep vent terminations clear of vegetation, snow, and ice.
• Schedule annual HVAC maintenance to clean inducer wheels and verify condensate drains.
• Replace pressure tubing every few years or during service visits if signs of wear appear.
• Install screening or a termination guard to reduce debris entry without restricting airflow.
• Address slow-draining condensate lines promptly to prevent trap overflow into the pressure circuit.

When To Call A Professional

Contact a licensed HVAC technician when the problem involves gas components, complex electrical diagnostics, control board replacement, or if the homeowner is uncomfortable with electrical testing. A pro will safely test gas valves, ignition sequence, and perform accurate inducer motor and control board diagnostics.

Resources And Further Reading

Refer to the Lennox installation and service manuals for model-specific pressure switch setpoints and wiring diagrams. Manufacturer technical bulletins and HVAC trade resources provide in-depth procedures for pressure switch and inducer testing methods. For safety information, consult local codes and utility guidelines.

Key Takeaways

Inducer Running But Pressure Switch Open typically points to venting restrictions, damaged tubing, condensate problems, a failed switch, or weak inducer performance. Systematic troubleshooting—visual inspection, vacuum measurement, continuity testing, and inducer checks—quickly isolates the cause. Never bypass safety switches, and seek professional service for gas or electrical repairs.