The article explains why a Lennox furnace ignites then shuts off, outlines step-by-step diagnostics and repair options, and highlights safety and maintenance practices to prevent recurrence. It focuses on actionable troubleshooting for homeowners and provides guidance on when to call a professional.
| Symptom | Common Cause | Typical Fix |
|---|---|---|
| Ignites Then Shuts Off Quickly | Restricted airflow, flame sensor failure, or blocked vent | Clean/replace filters, clean flame sensor, clear vents |
| Burner Lights Then Goes Out Repeatedly | Faulty ignitor, gas valve issue, or control board fault | Test/replace ignitor, inspect gas valve, check control board |
| Blower Runs But Heat Stops | Overheat limit switch tripping | Clean blower, ensure airflow, reset/replace limit switch |
Why A Lennox Furnace Ignites Then Shuts Off
When a Lennox furnace ignites then shuts off, the system is typically detecting a condition that prevents safe continuous operation. Modern furnaces include safety controls that will extinguish the flame if combustion or airflow parameters fall outside safe ranges.
Common systems involved include the flame sensor, pressure switch, limit switch, inducer motor, thermostat, gas valve, and the furnace control board. Problems can be mechanical, electrical, or related to airflow and venting.
Common Causes And How They Force A Shutdown
Dirty Or Faulty Flame Sensor
The flame sensor confirms that burners are producing flame. A flame sensor coated in soot or oxidation can fail to register flame, causing the control board to shut off the gas supply within seconds. Cleaning or replacing the flame sensor often resolves this symptom.
Restricted Airflow Or Clogged Filter
Restricted airflow raises heat exchanger temperatures quickly. The high-limit switch senses overheating and shuts the burners off as a protective measure. Replacing dirty filters and clearing returns can restore normal runtime.
Blocked Or Improper Venting
Blocked exhaust or intake vents can prevent the inducer motor from creating proper draft, or the pressure switch from closing. The furnace may ignite briefly before a pressure or safety switch causes shutdown.
Faulty Pressure Switch Or Inducer Motor
The pressure switch ensures the inducer motor establishes venting. If the inducer is weak or the pressure switch malfunctions, the furnace may light and then go out when the switch fails to confirm safe draft.
Ignitor Or Gas Valve Issues
A weak or intermittent ignitor may light briefly; a failing gas valve can cut fuel mid-cycle. Either condition can produce short ignition followed by shutdown and may require component replacement.
Control Board Or Wiring Problems
Faulty relays, scorched connectors, or intermittent signals on the control board can cause the burner to shut down prematurely. Loose or corroded wiring may also interrupt safety sensor readings.
Step-By-Step Diagnostic Checklist
Safety first: turn off power to the furnace at the switch or breaker before inspecting internal components. If any smell of gas or major damage exists, leave the area and call the gas utility or a qualified technician immediately.
- Confirm Symptom: Run several short cycles to observe whether the furnace consistently ignites then shuts off and note time between ignition and shutdown.
- Check Air Filter: Replace or clean the filter if dirty. A restricted filter is one of the simplest and most common causes.
- Inspect Vents: Check outside exhaust and intake terminations for nests, debris, snow, or blockage.
- Observe Inducer Motor: When the furnace starts, the inducer should run and sound consistent. A weak or noisy inducer can indicate failure.
- Test Pressure Switch: With the inducer running, the pressure switch should close. This may be visible as a small click or tested with a multimeter across switch terminals.
- Check Flame Sensor: Remove the flame sensor and clean gently with fine emery cloth or steel wool. Reinstall and test.
- Inspect Ignitor: Look for cracks or discoloration on hot surface ignitors. A resistance check can indicate failure.
- Monitor Error Codes: Lennox furnaces often flash LED codes on the control board. Compare flashes to the manual to pinpoint faults.
DIY Fixes Homeowners Can Safely Attempt
Replace Air Filter
Replacing an air filter is a low-risk, high-reward step. Use the manufacturer-recommended size and MERV rating to maintain airflow and indoor air quality.
Clean The Flame Sensor
Cleaning the flame sensor can restore proper flame detection. Turn power off, remove the sensor, gently clean the metal rod, and reinstall. Tighten gently and restore power to test.
Clear Vents And Drain Lines
Remove debris from outside vent terminations and make sure PVC intake pipes for high-efficiency furnaces are unobstructed. Check condensate drains for blockages and clear as needed.
Reset The Furnace
After basic maintenance, a power cycle may clear temporary faults. Turn off power at the switch for 30 seconds, then restore power. Observe the next ignition cycle for improvement.
When Professional Service Is Required
Call a licensed HVAC technician if the problem persists after basic checks or if any of the following are present: gas odor, persistent error codes, cracked heat exchanger, repeated inducer or pressure switch failure, or uncertain electrical issues.
Reasons to hire a pro: safe handling of gas valves and burners, accurate advanced diagnostics, replacement of sealed combustion components, and legal compliance with local codes.
Parts, Costs, And Typical Repair Times
| Component | Typical Replacement Cost (Parts) | Labor Time |
|---|---|---|
| Air Filter | $10–$60 | 10–20 minutes |
| Flame Sensor | $15–$80 | 15–45 minutes |
| Ignitor | $40–$250 | 30–60 minutes |
| Pressure Switch | $25–$150 | 30–60 minutes |
| Inducer Motor | $150–$500 | 1–2 hours |
| Control Board | $150–$600 | 1–2 hours |
Costs vary by furnace model, region, and whether the unit is under warranty. Labor rates depend on local HVAC companies; emergency service calls typically cost more.
Preventive Maintenance To Avoid Future Shutdowns
Schedule annual professional tune-ups before the heating season to inspect combustion, clean burners, verify heat exchanger integrity, and test safety devices. Regular maintenance reduces unexpected shutdowns and extends furnace life.
Monthly To Seasonal Tasks: Replace filters monthly or per manufacturer recommendations, keep vents and returns unobstructed, ensure outdoor vent terminations remain clear, and maintain adequate clearance around the furnace.
Safety Tips And Best Practices
- Turn off power before working inside the furnace and follow lockout/tagout where available.
- If a gas odor appears, leave the home immediately and call the gas company or 911 from a safe location.
- Do not attempt to modify gas piping or burners; only licensed professionals should perform gas-related repairs.
- Keep flammable materials away from the furnace and maintain proper clearance.
How Lennox Error Codes And LED Indicators Help
Lennox furnaces use LED flash codes to indicate specific faults. Check the furnace access panel for the control board LED and compare the flash pattern to the manual. This information can narrow the issue to sensors, safety switches, or control faults and guide targeted repairs.
Tip: Photograph the LED pattern or write down the flash count for the technician to expedite service.
Frequently Asked Questions
Will a dirty flame sensor always cause the furnace to shut off?
Not always, but a dirty or corroded flame sensor is one of the most common causes of a furnace that ignites then shuts off quickly. Cleaning the sensor often restores normal operation.
Can a thermostat cause this symptom?
A thermostat is less commonly the direct cause of short ignition cycles but can produce irregular calls for heat or rapid cycling if faulty. Verify thermostat settings and, if concerned, test with a basic wired thermostat or call a technician.
Is it safe to keep restarting the furnace when it shuts off?
Repeated restarts can stress components and may hide an underlying safety issue. It is better to run a few controlled cycles while troubleshooting and then seek professional service if the problem persists.
Final Guidance For Homeowners
When a Lennox furnace ignites then shuts off, start with basic airflow and flame sensor checks. Use LED codes and simple diagnostics to narrow the fault. Replace filters, clean the flame sensor, clear vents, and observe the inducer and pressure switch behavior. If the issue remains unresolved or if there are signs of gas leaks, electrical faults, or a cracked heat exchanger, contact a licensed HVAC professional promptly.
Key Takeaway: Many causes are simple maintenance items, but safety-related components and gas controls require professional diagnosis and repair to ensure safe, reliable operation.