Furnace Lights Then Shuts Off: Causes, Troubleshooting, and Fixes

The article explains why a furnace lights and then shuts off, common causes, step-by-step troubleshooting, safety checks, maintenance tips, and when to call a professional technician.

Symptom	Common Causes	Typical Fix
Furnace Lights Then Shuts Off Immediately	Dirty Flame Sensor, Faulty Ignitor	Clean/Replace Sensor, Replace Ignitor
Runs For A Few Seconds Then Stops	Pressure Switch, Vent Blockage, Clogged Filter	Clear Vent, Replace Filter, Test Switch
Intermittent Cycling	Thermostat Issues, Gas Valve Problems	Calibrate/Replace Thermostat, Inspect Valve

Why A Furnace Lights Then Shuts Off

When a furnace lights and then shuts off, it usually indicates the burner ignites but a safety or control device detects a fault and interrupts operation. Safety controls such as the flame sensor, pressure switch, limit switch, or a failing gas valve commonly trigger this behavior to prevent unsafe combustion or equipment damage.

Most Common Causes

Dirty Or Failing Flame Sensor

The flame sensor confirms the burner flame is present. A thin layer of corrosion or carbon can prevent the sensor from detecting flame, causing the control board to shut off the gas. This is the most frequent cause when the furnace lights then shuts off quickly.

Faulty Ignitor Or Pilot System

A weak ignitor may light briefly then fail, or a pilot assembly in older systems may not maintain flame. Intermittent ignition can lead to immediate shutdowns because the control verifies sustained combustion.

Pressure Switch Or Venting Problem

The pressure switch ensures the inducer motor and venting system create proper draft. If the pressure switch doesn’t close—due to a blocked vent, clogged condensate drain, or failing inducer—the furnace will light then shut off.

Limit Switch Or Overheating

A clogged air filter, restricted airflow, or blower failure can cause the heat exchanger to overheat. The limit switch will open to protect the system and force the burners off shortly after ignition.

Thermostat Or Control Board Issues

A malfunctioning thermostat, faulty wiring, or a failing control board can send incorrect signals, causing the furnace to start and then stop. Sporadic communication issues may present as brief ignition followed by immediate shutdown.

Gas Supply Or Gas Valve Problems

Insufficient gas pressure, interrupted supply, or a defective gas valve can allow a brief flame then closure. If the valve closes due to an internal fault or safety trip, the burner will extinguish.

Step-By-Step Troubleshooting

Safety First

Turn off power to the furnace at the switch and shut off gas if smell of gas is present. If gas odor or suspected leak exists, leave the home and call the gas company or emergency services. Never attempt repairs when a gas leak is suspected.

Visual And Basic Checks

• Check furnace error codes on the control board LED and consult the manual.
• Ensure thermostat settings are correct and batteries are fresh.
• Inspect the air filter; replace if dirty.
• Confirm supply and return vents are open and unobstructed.

Clean The Flame Sensor

Removing and gently cleaning the flame sensor with fine-grit sandpaper or steel wool often resolves the issue. Reinstall the sensor and test the furnace. This is a common DIY fix when the furnace lights then shuts off quickly.

Inspect The Ignitor

On electronic ignition systems, examine the hot surface ignitor for cracks or wear. A brittle or visibly damaged ignitor should be replaced. Handle the ignitor carefully; oils from hands can shorten life.

Check The Pressure Switch And Venting

Verify the inducer motor runs and listen for the vent draft. Inspect vent outdoors for bird nests, debris, or snow blockage. Test the pressure switch hose for cracks or obstructions. Replacing a defective pressure switch requires diagnostic tools and may need a pro.

Test The Limit Switch And Blower

If airflow problems are suspected, confirm the blower motor runs properly and bearings are not seized. Replace dirty filters and clean blower assembly. If the limit switch trips repeatedly, it may be defective or responding to real overheating—both require inspection.

Check Gas Supply And Valve Operation

Confirm other gas appliances work to rule out supply interruptions. Observe the gas valve during startup if qualified; a valve that closes after a few seconds could indicate internal failure or a control issue.

Thermostat Diagnostics

Swap with a known-good thermostat or bypass the thermostat safely to see if the furnace stays lit. Verify wiring connections and check for shorted or corroded wires at the furnace control board.

When To Call A Professional

If basic cleaning and filter replacement don’t stop the furnace lights then shuts off behavior, a trained HVAC technician is recommended for safety and accurate diagnosis. Call a professional when gas components, pressure switches, inducer motors, control boards, or combustion problems are suspected.

Hire A Technician For:

• Gas valve replacement or adjustment.
• Pressure switch, inducer motor, or control board diagnostics.
• Heat exchanger inspection for cracks or leaks.
• Complex electrical troubleshooting or component calibration.

Maintenance Tips To Prevent Recurrence

• Replace or clean the air filter every 1–3 months depending on use and home conditions.
• Schedule annual HVAC tune-ups to clean burners, sensors, and inspect components.
• Keep vents, flue, and outdoor intake clear of debris, snow, and nests.
• Use a carbon monoxide detector near sleeping areas and test it monthly.
• Record observed error codes or patterns to share with a technician for quicker diagnosis.

Diagnostic Checklist For Technicians

Component	Test	Expected Result
Flame Sensor	Clean/measure microamp signal	Stable flame detection current
Ignitor	Visual/crack test, resistance check	Proper glow and continuity
Pressure Switch	Measure closure with inducer on	Switch closes when proper vacuum present
Gas Valve	Voltage & operation test	Valve opens and maintains gas flow
Control Board	Read codes, trace signals	Correct sequencing and no fault codes

Cost Estimates For Repairs

Prices vary regionally. Typical ranges include sensor cleaning or replacement: $50–$150, ignitor replacement: $150–$400, pressure switch or inducer motor repairs: $200–$700, and control board replacement: $300–$900. Heat exchanger repair or replacement can be significantly higher and may prompt furnace replacement.

Safety Considerations And Carbon Monoxide

If a furnace cycles improperly, incomplete combustion could increase carbon monoxide risk. Ensure carbon monoxide detectors are installed and operational. Any detection of CO requires immediate evacuation and professional service.

FAQs

Can A Dirty Filter Cause The Furnace To Light Then Shut Off?

Yes. A clogged filter restricts airflow, causing overheating and limit switch trips that shut the burners off shortly after ignition.

Is It Safe To Clean The Flame Sensor Myself?

Yes, cleaning the flame sensor is a common homeowner task if comfortable with basic safety and power-off procedures. If not comfortable, contact a technician.

How Long Will A Repair Take?

Simple fixes like cleaning a flame sensor often take under an hour. Replacing parts like an inducer motor or control board can take 1–3 hours depending on accessibility and diagnosis time.

Additional Resources

Consult the furnace manufacturer’s manual, local utility safety guidance, and EPA or CPSC resources for carbon monoxide and gas safety. Keep documentation of repairs and error codes to aid future troubleshooting.

If a furnace lights then shuts off repeatedly, following these diagnostic steps and safety guidelines helps identify likely causes and determine whether a professional HVAC technician is needed.