Gas Furnace Pilot Light Won’t Stay Lit: Causes, Troubleshooting, and Solutions

When a gas furnace pilot light won’t stay lit, heating stops and safety concerns rise. This article explains common causes, step-by-step troubleshooting, safety precautions, repair costs, and maintenance tips so homeowners can address the problem effectively or know when to call a professional. Follow safety guidance and shut off the furnace if smelling gas or if unsure about any step.

Symptom	Possible Cause	Likely Fix
Pilot lights then goes out	Faulty thermocouple or draft	Clean or replace thermocouple; seal drafts
No pilot ignition	Clogged pilot or gas supply issue	Clean pilot or check gas valve/line
Yellow or weak flame	Incomplete combustion or contamination	Adjust air shutter or clean pilot assembly

How A Gas Furnace Pilot Light Works

The pilot light is a small, continuous flame that ignites the furnace main burners. Modern furnaces may use an electronic ignition, but older units rely on a standing pilot. The pilot flame heats a thermocouple or thermopile sensor that signals the gas valve to stay open. If the pilot goes out, the sensor cools and closes the valve for safety.

Common Reasons The Pilot Light Won’t Stay Lit

Understanding why a gas furnace pilot light won’t stay lit helps narrow down solutions quickly. Common causes include a failing thermocouple, drafts, dirty pilot orifice, gas supply problems, and improper flame shape or color. Less common causes include a damaged pilot tubing or issues with the gas control valve.

Faulty Or Worn Thermocouple Or Thermopile

The thermocouple senses the pilot flame and keeps the gas valve open. A weak or failing thermocouple cannot generate enough voltage, causing the valve to close and the pilot to go out. Thermocouple failure is one of the most frequent reasons a pilot light won’t stay lit.

Drafts And Airflow Disturbances

Drafts from open windows, vents, or gaps in the flue can blow out the pilot flame or alter its shape so the thermocouple is not heated properly. Windy conditions or exhaust backdrafts may intermittently extinguish the pilot.

Clogged Or Dirty Pilot Orifice

Debris, spider webs, soot, or dust can partially block the pilot orifice, creating a weak or unstable flame. A small, yellow, or flickering flame often indicates a dirty pilot or incomplete combustion.

Gas Supply Problems Or Valve Issues

If the gas pressure is too low or the gas valve is malfunctioning, the pilot may not receive a steady fuel supply. This can be due to a closed or partially closed supply valve, a defective control valve, or utility service interruptions.

Improper Flame Shape Or Color

A healthy pilot flame should be steady and primarily blue with a small inner cone. A yellow, lazy, or flickering flame suggests contamination, low gas pressure, or venting issues that can cause the flame to go out.

Safety Precautions Before Troubleshooting

Always prioritize safety. If a strong gas odor exists, leave the house immediately and call the gas utility or 911. Turn off gas at the main valve before inspecting components. Use a flashlight rather than open flames. If the user is uncomfortable or unsure, stop and call a licensed HVAC technician.

Step-By-Step Troubleshooting Guide

These steps help identify why a gas furnace pilot light won’t stay lit. Perform simple checks first and stop if the situation seems unsafe or beyond homeowner skills.

1. Check For Gas Odor: If present, evacuate and call emergency services. Do not attempt repairs.
2. Verify Gas Supply: Ensure the furnace gas shutoff and main gas valve are open. Confirm other gas appliances work.
3. Inspect Pilot Flame: Observe the pilot flame color and stability. A strong blue flame indicates proper combustion.
4. Look For Drafts: Close windows and doors, check vents, and seal gaps around the furnace chamber to reduce drafts.
5. Clean The Pilot Orifice: Turn off gas, remove the pilot tube if accessible, and use compressed air or a thin wire to clear debris. Reassemble and relight per manufacturer instructions.
6. Test The Thermocouple: Ensure the thermocouple tip sits in the pilot flame. If corrosion or bending is present, consider replacement. For testing, a multimeter can measure millivolts when heated.
7. Check The Gas Control Valve: If the valve sticks or fails to supply steady gas, a professional repair or replacement is likely required.
8. Inspect Venting And Flue: Blocked or damaged venting can cause backdrafts and unstable pilot flames; repair as needed.

How To Clean Or Replace A Thermocouple

Cleaning or replacing the thermocouple is a common homeowner task when comfortable with basic tools. Always shut off gas first and follow the furnace manual.

• Turn Off The Gas And Power: Close the gas valve and switch off electrical power to the furnace.
• Remove Access Panels: Locate pilot assembly and thermocouple where the pilot flame contacts the sensor tip.
• Clean Gently: Wipe the thermocouple tip with fine sandpaper or emery cloth to remove soot or oxidation.
• Reposition Or Replace: Ensure the thermocouple tip is correctly positioned in the flame. Replace the thermocouple if visibly damaged or if cleaning does not restore operation.
• Tighten Connections And Test: Reattach the thermocouple to the gas valve, restore gas and power, and light the pilot per instructions.

When To Call A Professional

If basic cleaning and thermocouple replacement do not resolve the issue, professional service is recommended. Call an HVAC technician if there are persistent gas smells, suspected gas line problems, a faulty gas control valve, or flue and venting issues. Licensed technicians have specialized tools and safety training.

Estimated Repair Costs And Parts

Costs vary by region and furnace type. Typical estimates: thermocouple replacement ranges from $75 to $200 including parts and labor. Cleaning and diagnostic service calls often range from $80 to $200. Replacing a gas control valve or pilot assembly can cost $300 to $900. Always get multiple quotes and confirm licensing and insurance.

Preventive Maintenance To Keep The Pilot Lit

Regular maintenance reduces the chance a gas furnace pilot light won’t stay lit. Recommended steps include annual professional tune-ups, cleaning the pilot assembly, checking and sealing drafts, and inspecting venting and flue pipes for blockages or corrosion.

• Schedule Annual HVAC Inspections And Cleanings
• Replace Filters Regularly To Reduce Dust
• Keep The Area Around The Furnace Clear To Prevent Drafts
• Address Any Moisture Or Corrosion Near The Furnace Promptly

Common Myths And Misconceptions

Myth: “A yellow pilot flame is always harmless.” In reality, a yellow flame often indicates incomplete combustion and requires attention to prevent carbon monoxide issues. Myth: “If the pilot keeps going out, only the pilot light needs to be relit.” Repeated relighting without diagnosing the cause can be dangerous; persistent issues usually indicate a deeper problem.

Frequently Asked Questions (FAQs)

How Long Should A Pilot Light Stay Lit?

A pilot light should remain lit continuously in systems that use a standing pilot. If it goes out frequently, that signals a malfunction such as a failing thermocouple, draft, or obstruction.

Is It Safe To Keep Relighting The Pilot?

Occasional relighting is acceptable following manufacturer instructions, but repeated relighting without fixing the underlying cause is unsafe. Persistent problems warrant inspection by a qualified technician.

Can Cold Weather Cause The Pilot To Go Out?

Yes. Cold temperatures can increase drafts or cause contraction in components, altering the flame shape or thermocouple position. Proper sealing and routine maintenance reduce weather-related issues.

What If The Furnace Uses An Electronic Ignition?

Electronic ignition systems do not have a standing pilot. If the furnace won’t stay lit or light properly, problems likely involve the igniter, flame sensor, control board, or gas valve, and professional diagnosis is recommended.

Resources And Further Reading

For safety and detailed repair procedures, consult the furnace owner’s manual, the local gas utility safety guidelines, and EPA or CPSC resources on indoor air quality and combustion appliance safety. When in doubt, contact a licensed HVAC contractor for inspection and repairs.