Why a Gas Furnace Ignites Then Goes Out

Many homeowners encounter a frustrating problem when their gas furnace ignites then goes out shortly afterward. This behavior can indicate a range of issues from simple sensor faults to more serious safety or combustion problems. The following article explores common causes, step-by-step troubleshooting, safety checks, typical repair costs, and preventive maintenance to help diagnose and resolve the issue.

Symptom	Likely Causes	Typical Fix
Furnace Lights Then Shuts Off	Faulty flame sensor, dirty burner, limit switch tripping	Clean/replace sensor, clean burners, check airflow
Short Ignition Cycles	Pilot or ignition system issues, pressure switch fault	Inspect ignition system, check venting, replace switch
Ignites Then Immediately Shuts Off	Blocked flue, flame rollout switch, insufficient gas pressure	Clear vents, service heat exchanger, call technician

How A Gas Furnace Ignition Sequence Works

Understanding the normal ignition sequence helps pinpoint where the failure occurs. When the thermostat calls for heat, the furnace activates the inducer motor and ignition system, opens the gas valve, and the burners ignite. A flame sensor or flame rectification system confirms combustion before the gas valve stays open.

If the furnace lights and then goes out, the control board or safety devices likely detect a problem and shut the gas off to prevent hazards. Interruption in any part of this sequence can cause the furnace to extinguish after ignition.

Common Causes When A Gas Furnace Ignites Then Goes Out

Dirty Or Faulty Flame Sensor

The flame sensor detects the presence of a flame and signals the control board to keep the gas valve open. A dirty or corroded sensor often fails to register flame even though the burners ignite, so the furnace shuts down soon after lighting.

Cleaning the flame sensor with fine emery cloth or replacing it if worn is a common and cost-effective fix.

Pilot Or Ignition Problems

For furnaces with a standing pilot, a weak pilot flame or misaligned thermocouple can cause shutdowns. For electronic ignition systems, a failing hot surface igniter or intermittent spark can trigger short cycles.

Ignition components may require cleaning, repositioning, or replacement to restore reliable operation.

Blocked Exhaust Or Venting Issues

Blockages in the flue, chimney, or venting system can cause combustion gases to accumulate, prompting pressure sensors or rollout switches to shut down the furnace. Birds, leaves, snow, or debris commonly obstruct vents.

Clearing vents and ensuring proper draft is essential; prolonged blockage risks carbon monoxide buildup and requires immediate attention.

Limit Switch Or Overheating

The high-limit switch prevents overheating by shutting off the burner if internal temperatures exceed safe levels. Restricted airflow from dirty filters, closed vents, or blower issues can overheat the heat exchanger and cause repeated shutdowns.

Replacing filters and ensuring blower operation often resolves limit switch trips; a failing switch or control may need replacement if overheating is not present.

Gas Supply Or Pressure Problems

Insufficient gas pressure, a partially closed gas valve, or problems with the gas regulator can cause the burners to extinguish after ignition. Appliances on the same gas line can reveal pressure issues when multiple devices underperform simultaneously.

A licensed technician should test gas pressure and inspect regulators and valves when gas supply is suspected.

Faulty Pressure Switch Or Inducer Motor

The pressure switch confirms proper venting and inducer motor operation before allowing ignition. If the inducer fails to build required draft or the pressure switch is out of calibration, the furnace may start then shut off quickly.

Technicians test switch continuity and inducer performance and replace components as needed to restore reliable draft confirmation.

Flame Rollout Switch Or Heat Exchanger Issues

The flame rollout switch trips when flames escape the combustion chamber due to a cracked heat exchanger or severe blockage. This safety device will shut off the furnace immediately after ignition to prevent dangerous conditions.

Heat exchanger cracks are serious and often require furnace replacement; a qualified technician must evaluate and repair such conditions.

Troubleshooting Steps Homeowners Can Safely Perform

Visual And Odor Checks

First, perform a quick visual inspection for obvious issues: blocked outdoor vents, heavy soot around burners, and the presence of a natural gas odor. If gas smell is detected, evacuate and call the gas utility immediately.

Check And Replace Air Filters

A dirty filter restricts airflow, causing overheating and limit trips. Replacing a clogged filter is a simple action that can restore normal operation.

Inspect And Clean The Flame Sensor

Turn off power and gas, remove the flame sensor, and gently clean the metal rod with fine emery cloth. Reinstall and test the furnace. Cleaning often resolves ignition-shutdown cycles related to sensor contamination.

Ensure Vents And Chimney Are Clear

Check the outdoor vent termination for debris or nests. If accessible and safe, clear visible blockages. Do not attempt to clear internal flue blockages without professional tools and experience.

Reset And Observe The Furnace

Many furnaces have a reset or diagnostic sequence. Consult the owner’s manual for LED codes and reset procedures. Observe the furnace through a full cycle to note any error codes or patterns when it ignites then goes out.

When To Call A Professional Technician

If basic homeowner checks don’t resolve the problem, or if the fault involves gas pressure, a cracked heat exchanger, internal wiring, or persistent carbon monoxide concerns, a licensed HVAC technician should be contacted.

Call a professional immediately if there is a gas odor, visible corrosion or cracks in the heat exchanger, repeated safety switch trips, or if multiple appliances show gas supply issues.

Safety Checks And Carbon Monoxide Considerations

Any unexpected furnace shutdowns can correlate with incomplete combustion and increased carbon monoxide risk. Ensure the home has functioning carbon monoxide detectors near sleeping areas and in the furnace room if applicable.

Keep the air supply and flue clear, and avoid operating the furnace if carbon monoxide detectors trigger or if there is persistent soot and stale air smells.

Common Diagnostic Codes And What They Mean

Many modern furnaces display LED error codes that indicate specific faults such as flame loss, pressure switch failure, limit trips, or ignition lockouts. Consult the furnace’s error code chart to interpret these signals.

When the furnace ignites then goes out, common codes include flame failure or limit switch activations; technicians use these codes to prioritize checks of the flame sensor, gas valve, venting, and airflow.

Estimated Repair Costs And Typical Parts

Issue	Typical Cost Range (Parts + Labor)
Flame Sensor Cleaning/Replacement	$50 – $250
Igniter Replacement	$150 – $400
Pressure Switch Replacement	$150 – $350
Inducer Motor Replacement	$300 – $700
Heat Exchanger Repair/Replacement	$800 – $3,500 (often replacement)

Prices vary significantly by region, furnace model, accessibility, and whether the unit is under warranty. Always obtain multiple estimates for major repairs and confirm technician licensing and insurance.

Preventive Maintenance To Avoid Ignition Shutdowns

• Schedule annual professional tune-ups to inspect combustion, gas pressure, heat exchanger integrity, and controls.
• Replace air filters every 1–3 months to prevent airflow restriction.
• Keep outdoor vents and intake areas clear of debris, snow, and nests.
• Install and test carbon monoxide detectors and change batteries regularly.
• Address unusual noises, odors, or frequent short cycles promptly to prevent escalation.

Frequently Asked Questions

Can a dirty flame sensor cause the furnace to light then go out?

Yes. A contaminated flame sensor is one of the most common causes of a furnace that ignites then shuts off, because it fails to detect flame and prompts the control board to close the gas valve.

Is it dangerous to run the furnace if it keeps shutting off?

Repeated short cycling can stress components and may indicate dangerous conditions like blocked vents or a cracked heat exchanger. Do not ignore recurring shutdowns; arrange for inspection if basic troubleshooting fails.

How long should a furnace run once it ignites?

A healthy furnace typically runs for several minutes to reach thermostat setpoint, depending on home size and thermostat settings. Immediate shutdown within seconds to a minute usually signals a control, sensor, or venting issue.

Will replacing a furnace be necessary if it ignites then goes out?

Not always. Many causes are repairable, such as sensors, igniters, or vents. Replacement becomes more likely with a cracked heat exchanger, repeated major failures, or when repair costs approach the value of a new unit.

Checklist For Communicating With A Technician

1. Document the sequence: how long the furnace runs after ignition and any error codes or LED flashes.
2. Note recent changes: new appliances, weather events, or service history.
3. Report any odors, soot, or carbon monoxide detector activity.
4. Provide the furnace make, model, and age, and any prior repairs.

Clear information helps technicians diagnose the cause of a furnace that ignites then goes out more efficiently, saving time and cost.

For safety-critical concerns or unresolved issues after basic checks, homeowners should prioritize a licensed HVAC professional to inspect combustion safety elements and gas-related components.