Furnace Pilot Light: How It Works, Common Problems & How to Fix It
A furnace pilot light is a small, continuously burning flame that ignites the main burner in older gas furnaces. When it goes out, your furnace stops producing heat. Understanding how your pilot light works—and how to safely relight or troubleshoot it—can save you time, money, and a cold night at home.
What Is a Furnace Pilot Light?
A furnace pilot light is a small standing flame located inside your gas furnace that serves as the ignition source for the main burner. When your thermostat calls for heat, gas flows to the burner and the pilot light ignites it, producing the heat that warms your home.
Pilot lights are found in older, traditional gas furnaces manufactured before the mid-1990s. Modern furnaces typically use electronic ignition systems—such as intermittent pilot or hot surface ignition—instead of a standing pilot light. If your furnace is newer and not producing heat, the issue is likely with the furnace igniter, not a pilot light.
How Does a Furnace Pilot Light Work?
A furnace pilot light works by maintaining a continuous small flame near the burner assembly. Here's the step-by-step process:
- The thermostat signals for heat. When the temperature in your home drops below the set point, your thermostat sends a signal to the furnace.
- Gas flows to the burner. The furnace gas valve opens, allowing fuel to reach the main burner.
- The pilot light ignites the burner. The standing flame from the pilot light ignites the gas at the burner, generating heat.
- The thermocouple monitors the flame. A safety device called a thermocouple detects whether the pilot light is burning. If it goes out, the thermocouple shuts off the gas supply to prevent dangerous gas buildup.
Learn more about how a furnace works.
Why Does a Furnace Pilot Light Go Out?
A furnace pilot light can go out for several reasons, ranging from minor airflow issues to faulty components. The most common causes include:
- Drafts or air currents near the furnace that blow out the flame
- A dirty or clogged pilot orifice that restricts gas flow
- A faulty thermocouple that incorrectly signals the gas valve to close
- Low gas pressure from the supply line
- A malfunctioning gas valve that fails to deliver consistent fuel
A one-time outage is usually not a cause for concern. If your pilot light goes out repeatedly, a faulty thermocouple or dirty orifice is the most likely culprit and should be inspected by a licensed HVAC technician.
How to Relight a Furnace Pilot Light
Relighting a furnace pilot light is a straightforward process that most homeowners can do safely. Before you begin, ensure there is no smell of gas in your home. If you detect a gas odor, do not attempt to relight the pilot—leave your home immediately and contact your gas provider.
What you'll need: A long-reach lighter or fireplace matches.
Step-by-step instructions:
- Turn off the furnace. Set your thermostat to the lowest setting and locate the furnace's power switch. Turn it off.
- Find the pilot light assembly. Open the furnace access panel. The pilot assembly is typically near the bottom of the unit, near the burner.
- Locate the gas valve control knob. It will have settings labeled "On," "Off," and "Pilot."
- Turn the knob to "Off." Wait at least 3–5 minutes to allow any residual gas to dissipate before proceeding.
- Turn the knob to "Pilot." Press and hold the knob down to allow gas to flow to the pilot.
- Ignite the pilot. While holding the knob down, bring your lighter or match to the pilot opening and ignite the flame.
- Hold the knob for 20–30 seconds. This heats the thermocouple, signaling it to keep the gas valve open.
- Release the knob slowly. If the flame stays lit, turn the knob to "On" and restore power to the furnace.
- Test your furnace. Raise the thermostat above the current room temperature to confirm the main burner ignites.
If the pilot light won't stay lit after two or three attempts, stop and contact a licensed HVAC technician for professional furnace service. Do not continue attempting to relight it.
Furnace Pilot Light Troubleshooting: Common Problems & Solutions
Problem | Likely Cause | Recommended Action |
|---|---|---|
Pilot light won't light | Clogged orifice or no gas supply | Clean orifice; check gas supply |
Pilot light won't stay lit | Faulty thermocouple | Replace thermocouple |
Pilot light keeps going out | Draft or low gas pressure | Seal drafts; call gas provider |
Weak or yellow pilot flame | Dirty orifice or incorrect air mix | Clean orifice; call technician |
No pilot light at all | Gas valve failure | Call a licensed HVAC technician |
What Does a Healthy Pilot Light Look Like?
A healthy furnace pilot light should burn with a steady, blue flame with a small yellow tip. A mostly yellow or orange flame indicates incomplete combustion—often caused by a dirty orifice or improper air-to-gas ratio—and should be addressed by a professional. A flickering or weak flame suggests a draft or low gas pressure issue.
Thermocouple vs. Thermopile: What's the Difference?
Both components are safety devices that monitor the pilot flame, but they serve slightly different roles.
- A thermocouple generates a small electrical current when heated by the pilot flame. This current signals the gas valve to remain open. If the pilot goes out, the current stops and the gas valve closes automatically. Thermocouples are the most common safety device in older furnaces with standing pilot lights.
- A thermopile is a more powerful version of the thermocouple, generating enough electricity to power a gas valve without a separate power source. Thermopiles are better suited for furnaces or appliances that require more electrical output to operate the valve.
If your pilot light won't stay lit, a worn thermocouple is the most common cause and is generally inexpensive to replace.
When to Call a Professional for Furnace Pilot Light Issues
Homeowners can safely relight a furnace pilot light under normal conditions. However, contact your local Carrier dealer if:
- You smell gas at any point during the process
- The pilot light repeatedly goes out after relighting
- The pilot flame appears yellow, weak, or irregular
- The furnace makes unusual noises after the pilot is relit
- You are unsure of any step in the relighting process
Gas appliances carry inherent safety risks. When in doubt, a professional furnace inspection is always the safer and more cost-effective choice in the long run.
Frequently Asked Questions
Furnaces manufactured before the mid-1990s typically use a standing pilot light. Newer furnaces use electronic ignition systems, such as a hot surface igniter or intermittent pilot. To check, look inside the furnace access panel—if you see a small, continuously burning flame, your furnace has a pilot light. If there is no standing flame and the furnace uses a control board, it likely has electronic ignition.
Yes, relighting a furnace pilot light is generally safe if done correctly and only when there is no smell of gas. Always wait at least 3–5 minutes after turning the gas off before attempting to relight, use a long-reach lighter, and follow the manufacturer's instructions printed on the furnace. If you detect gas or the pilot won't stay lit after a few attempts, contact a professional.
Thermocouple replacement typically costs between $150 and $250 when performed by a licensed HVAC technician, including parts and labor. The part itself costs between $5 and $30. This is one of the more affordable furnace repairs and is a common fix when a pilot light repeatedly fails to stay lit.
No. In furnaces that rely on a standing pilot light, the pilot flame is required to ignite the main burner. Without it, the furnace cannot produce heat. If your pilot light goes out, your furnace will not operate until the pilot is relit or the underlying issue is resolved.
A properly functioning furnace pilot light should stay lit continuously throughout the heating season. An occasional outage due to a strong draft may be normal, but a pilot light that goes out frequently—more than once or twice per season—typically signals a problem with the thermocouple, gas pressure, or pilot orifice that should be inspected.
About The Author: Travis Baugh is a Digital Brand Marketing Manager for Carrier, where he develops informative, straightforward content to help homeowners better understand heating, cooling, and indoor air quality. His writing is focused on empowering homeowners to make confident, well-informed choices about their home comfort systems.
About The Expert Reviewer: Seth Faber is a Product Manager for Residential Furnaces at Carrier Corporation, where he helps develop dependable, high-performance heating solutions that deliver reliable comfort while meeting evolving homeowner needs, energy efficiency standards, and industry regulations.