Lennox Furnace Ignites Then Shuts Off: Causes, Diagnostics, and Fixes

The Lennox furnace that ignites then shuts off is a common issue that can stem from several components, safety systems, or airflow problems. This article explains likely causes, step-by-step diagnostics, DIY checks, and professional repair guidance to help homeowners identify and resolve the problem safely and efficiently.

Symptom	Possible Causes	Likely Fix
Ignites Briefly Then Shuts Off	Flame Sensing Issues, Dirty Flame Sensor, Thermostat Short Cycle	Clean/Replace Sensor, Check Thermostat Settings
Ignites, Runs Seconds, Then Shuts Off	Pressure Switch, Vent Blockage, Inducer Motor Failure	Inspect Venting, Test/Replace Switch or Motor
Ignites Several Times, Then Lockout	Gas Valve Problems, Control Board Faults, Overheating	Check Gas Supply, Control Board Diagnostics, Replace Parts

How Lennox Furnaces Operate And Why They Shut Off

Lennox furnaces follow a controlled ignition sequence: the thermostat calls for heat, the inducer motor starts, pressure and safety switches verify venting, the ignitor heats or spark is created, the gas valve opens, and the burner lights. The furnace continuously monitors flame and safety inputs. If any safety input fails or a sensor detects an unsafe condition, the furnace will shut off to protect the home and unit.

Common Causes When A Furnace Ignites Then Shuts Off

Flame Sensor Problems

The flame sensor detects that burners are lit; a weak or dirty sensor often causes the control board to interpret the flame as absent and close the gas valve. Dirty flame sensors are a leading cause of short run cycles after ignition.

Thermostat Or Control Settings and Short Cycling

Short cycling occurs when the furnace turns on and off too quickly. Incorrect thermostat wiring, a faulty thermostat, or an improperly sized furnace can cause frequent cycling that appears as ignition followed by shutdown.

Blocked Venting Or Pressure Switch Faults

If the inducer motor cannot establish proper draft due to blocked venting, or if the pressure switch that detects draft is faulty, the furnace may start the ignition sequence and then stop when the switch fails to close.

Overheating And Limit Switch Trips

Restricted airflow from dirty filters or closed vents can cause heat to build and trip the high-limit switch. This safety shuts the burners off until the unit cools, producing brief ignition cycles followed by shutdowns.

Gas Supply, Gas Valve, Or Ignitor Issues

Intermittent gas supply, failing gas valves, or weak/malfunctioning ignitors can lead to ignition attempts that end in shutdown. The control board may lock out after repeated unsuccessful ignitions for safety.

Faulty Control Board Or Safety Interlocks

Modern Lennox furnaces rely on a control board to manage sequences and detect faults. A failing board or intermittent sensor inputs can cause erratic cycling: ignition followed by immediate shutdown or lockouts.

Diagnostic Steps To Identify The Problem

Safety First

Turn off power and gas before opening panels for inspection. If the homeowner is uncomfortable, a licensed HVAC technician should be called. Improper handling of gas or electrical components can be dangerous.

Observe Furnace Behavior And Error Codes

Check the furnace LED status light or display for fault codes. Lennox furnaces provide diagnostic flash patterns; cross-reference the pattern with the manual to narrow down the issue.

Check Air Filter And Airflow

Inspect and replace the air filter if dirty. Ensure return vents are open and registers are not blocked. Restricted airflow is a common and easily corrected cause of short cycling.

Inspect And Clean The Flame Sensor

Locate the flame sensor (a metal rod near the burners). Remove and gently clean with fine-grit sandpaper or emery cloth to remove oxidation and buildup, then reinstall. This often resolves ignition-then-shutoff issues.

Test The Pressure Switch And Venting

Visually inspect vent piping for obstructions, ice, or bird nests. Listen for inducer motor operation. Use a multimeter or manometer to test the pressure switch function; it should close when the inducer creates draft.

Check The Ignitor And Gas Valve

Observe whether the ignitor glows or a spark is present and whether the burners light consistently. Use a multimeter to test ignitor resistance per Lennox specifications. Check that the gas valve is supplying consistent pressure; if not, confirm gas supply and regulator performance.

Examine The Limit Switch And Heat Exchanger

Inspect the high-limit switch for continuity when the unit is cool. Ensure the heat exchanger is not obstructed by corrosion or debris. Overheating due to a clogged exchanger or blocked flue can trip safety circuits.

DIY Fixes Homeowners Can Safely Try

Replace Or Clean The Air Filter

Replace disposable filters or clean reusable filters. Recommend filters rated MERV 8–11 for balancing efficiency and airflow. Regular monthly checks during heavy use can prevent many airflow-related shutdowns.

Clean The Flame Sensor

Disconnect power, remove the sensor, clean gently, and reinstall. This simple step often restores proper flame sensing and eliminates short cycling after ignition.

Clear Venting And Intake Pipes

From ground level, check for obstructions at the intake and exhaust terminations. Remove debris or nests, and ensure proper termination clearances to avoid pressure switch faults.

Reset The Furnace And Thermostat

Resetting power at the furnace and thermostat can clear temporary faults. After reset, monitor for repeated lockouts which indicate a persistent fault requiring deeper diagnostics.

When To Call A Professional HVAC Technician

Contact a licensed technician if cleaning and basic checks do not resolve the issue, if there is a gas smell, if control board diagnostics indicate complex faults, or if electrical components require replacement. Professionals have tools to test gas pressure, measure combustion, and safely replace sensitive parts.

Parts That Often Require Professional Replacement

• Flame Sensor — inexpensive but can be hard to reach in some models.
• Ignitor — fragile and must match model specifications.
• Pressure Switch or Inducer Motor — requires combustion testing.
• Gas Valve — professional gas handling and leak testing required.
• Control Board — proper diagnostics and programming necessary.

Maintenance Tips To Prevent Future Ignition And Shutdown Problems

• Schedule annual professional tune-ups to clean burners, test safety switches, and verify limits.
• Change or clean air filters monthly during heating season.
• Keep vent terminations clear of snow, ice, or debris in winter.
• Upgrade to a programmable or smart thermostat compatible with the Lennox system to avoid wiring or setting errors.
• Address HVAC airflow issues promptly by inspecting ductwork and ensuring returns are unobstructed.

Understanding Lennox Diagnostics And Error Codes

Lennox furnaces display LED flash codes or error messages on the control board or thermostat. Recording the exact pattern and number of flashes speeds diagnosis. The Lennox manual or a licensed technician can map codes to likely faults such as pressure switch, flame sense, limit trip, or gas valve failures.

Cost Estimates For Common Repairs

Repair	Typical Cost Range (Parts + Labor)
Clean/Replace Flame Sensor	$80–$250
Ignitor Replacement	$150–$400
Pressure Switch/Inducer Motor	$200–$700
Gas Valve Replacement	$300–$900
Control Board Replacement	$250–$900+

When Short Cycling Means It’s Time For A System Upgrade

If a furnace is repeatedly short cycling and repairs are frequent or costly, it may indicate mis-sizing, a failing heat exchanger, or an aging system. Evaluating the unit’s age, efficiency, and repair history helps determine whether replacement with a modern, efficient Lennox model is more cost-effective in the long term.

Key Takeaways For Homeowners

• Start with simple checks: filter, vents, and flame sensor cleaning.
• Record LED codes and furnace behavior; they guide diagnostics.
• Call professionals for gas-related, electrical, or control board issues.
• Maintain annual service to prevent recurring ignition/shutdown cycles.

For safety and accuracy, homeowners should rely on licensed HVAC technicians for complex diagnostics and gas-related repairs. Proper maintenance and prompt attention to short cycling can extend furnace life and maintain efficient, reliable heating.