Why a Furnace Pilot Won’t Stay Lit and How to Fix It

The pilot light on a gas furnace serves as a small, continuous flame that ignites the main burners when heat is needed. When the pilot light won’t stay lit, the furnace cannot run reliably, leaving a home cold and potentially creating safety or efficiency issues. This article explains common causes, diagnostic steps, and repair options for a pilot that won’t remain lit, with safety considerations and guidance on when to call a professional.

Common Cause	Symptoms	Possible Fix
Dirty Pilot Orifice	Weak or flickering flame	Clean orifice with compressed air or replace
Faulty Thermocouple	Pilot lights then dies after a few seconds	Test voltage; replace thermocouple
Pilot Adjustment	Blue inner flame missing	Adjust flame height or reposition
Gas Supply Issues	No gas flow or low pressure	Check valve, regulator, or call gas utility
Drafts Or Venting Problems	Pilot extinguishes intermittently	Seal drafts, inspect venting
Faulty Gas Valve Or Control	Pilot won’t stay lit despite good thermocouple	Replace valve/control assembly

How A Pilot System Works

A pilot system usually comprises a pilot burner, a thermocouple or thermopile, a gas valve, and related controls. The pilot flame heats the thermocouple, generating a small electrical signal that tells the gas valve to stay open. If the thermocouple cools, it signals the gas valve to shut off, extinguishing the pilot. Modern systems may use electronic ignition instead of a standing pilot.

Common Reasons The Pilot Light Won’t Stay Lit

Dirty Or Clogged Pilot Orifice

Dust, soot, spider webs, and other debris can block the pilot orifice, reducing gas flow and producing a weak or unstable flame. Symptoms include a flame that looks small, yellow, or flickers.

Faulty Or Weak Thermocouple

The thermocouple senses the pilot flame and keeps the gas valve open. A failing thermocouple may not generate enough millivolts to hold the valve open, so the pilot lights then goes out after a few seconds. This is a very common cause of intermittent pilot problems.

Poor Pilot Flame Adjustment Or Position

If the flame does not properly envelop the thermocouple tip, the device won’t heat enough to signal the gas valve to stay open. The correct pilot flame usually has a steady blue inner cone that contacts the thermocouple.

Drafts Or Venting Problems

Strong drafts from furnace draft inducer fans, open windows, attic access, or poor chimney draft can blow out or disturb the pilot flame. Venting issues may also cause combustion air imbalance, affecting flame stability.

Gas Supply Or Regulator Issues

Low gas pressure, a partially closed gas shutoff valve, or a faulty regulator can produce insufficient flow to sustain the pilot. A complete loss of gas supply will prevent ignition entirely.

Malfunctioning Gas Valve Or Controls

If the gas valve or control module itself is faulty, it may not remain open even when the thermocouple calls for gas. Electrical problems in the control circuit can produce similar symptoms.

Safety First: What To Do Before Troubleshooting

Before attempting any work, the furnace should be turned off and the gas supply secured if components are to be disassembled. If a gas odor is present, leave the building immediately and call the gas utility or 911. Working on gas appliances carries risk; homeowners should stop and call a licensed HVAC technician if they are uncomfortable or unsure.

Step-By-Step Troubleshooting Guide

1. Observe The Pilot Flame

Turn the furnace control to pilot mode and watch the pilot. A steady blue flame about 1/2 inch long that envelopes the thermocouple is ideal. A weak, yellow, or flickering flame indicates a problem with orifice cleanliness, gas supply, or drafts.

2. Check For Drafts And Venting Problems

Look for sources of drafts near the furnace and pilot assembly, including open doors, windows, nearby fans, attic or basement openings, and return vents. Temporarily closing openings may reveal if drafts are the cause. Inspect the flue and vent connectors for blockages or disconnections.

3. Inspect The Thermocouple

Visually inspect the thermocouple tip. It should be clean and positioned in the pilot flame. If it’s blackened or misaligned, gently clean it with fine emery cloth and reposition it so the flame fully surrounds the tip.

4. Test The Thermocouple

A multimeter can measure the thermocouple voltage. With the pilot lit, a healthy thermocouple typically produces about 25–30 millivolts. If the reading is significantly below the expected range, consider replacement.

5. Clean The Pilot Orifice

Turn off the gas and allow the area to cool. Remove the pilot assembly per manufacturer instructions and blow out the orifice with compressed air or use a thin wire designated for pilot cleaning. Avoid enlarging the orifice, which would alter gas flow characteristics.

6. Check Gas Supply And Valves

Ensure the gas shutoff valve is fully open and the supply to the furnace is on. If multiple appliances are affected, the issue may be with the household supply or regulator. Significant pressure issues or suspected leaks require a utility or licensed technician.

7. Evaluate The Gas Control Valve

If the thermocouple and pilot assembly are in good condition but the pilot still won’t stay lit, the gas control valve could be the culprit. Valve replacement should be performed by a trained technician since it involves sealed components and gas connections.

When To Replace Components

Thermocouples are inexpensive and a frequent point of failure; replacement is often a practical first repair. The pilot assembly orifice may also be replaced if cleaning does not restore a proper flame. A gas valve or control module replacement is costlier and usually justified after confirming other components are functional.

DIY Replacement Basics

For a homeowner comfortable with basic tools, replacing a thermocouple is a common DIY task. Typical steps include shutting off gas, disconnecting the thermocouple from the gas valve, removing the old unit, installing the new one, and testing. Always follow the furnace manufacturer’s instructions and local codes.

When To Call A Professional

Call a licensed HVAC technician or gas contractor when there is a gas leak, persistent pilot failure after basic checks, evidence of combustion issues, complex control or valve replacement, or uncertainty about safety. Professionals can test gas pressure, inspect the control circuit, and ensure safe, code-compliant repairs.

Preventive Maintenance To Keep The Pilot Lit

• Schedule annual furnace inspections and cleaning to remove debris that can clog the pilot orifice.
• Check and replace worn thermocouples during routine service intervals.
• Seal gaps and reduce drafts near the furnace location.
• Keep the area around the furnace clean and free of dust or pests.
• Upgrade to electronic ignition if frequent pilot problems occur—this eliminates a standing pilot and improves efficiency.

Costs And Time Estimates

Thermocouple replacement typically costs $20–$50 for parts plus $75–$200 labor if done by a technician. Cleaning the pilot orifice is low-cost if DIY. Gas valve or control replacements can range from several hundred to over a thousand dollars depending on the furnace model and parts. Professional diagnosis usually takes under an hour.

Electronic Ignition As An Alternative

Homes that frequently experience pilot issues may benefit from converting to intermittent pilot or hot-surface/electronic ignition systems. Benefits include improved energy efficiency, reduced gas use, and elimination of pilot-related outages. Conversion cost varies by furnace compatibility and labor.

Common Misconceptions

• Myth: The pilot light is purely decorative. Fact: It is a critical ignition source for many furnaces and must function correctly for safe operation.
• Myth: If the pilot lights once, the thermocouple is fine. Fact: A marginal thermocouple may pass briefly but fail to sustain operation.
• Myth: Any yellow flame is acceptable. Fact: A yellow or sooty flame typically indicates incomplete combustion or contamination and warrants inspection.

Troubleshooting Checklist For Technicians

1. Confirm pilot flame characteristics and thermocouple position.
2. Measure thermocouple voltage with pilot lit.
3. Inspect and clean pilot orifice and assembly.
4. Verify gas supply, pressure, and shutoff valves.
5. Test gas control valve operation and continuity.
6. Check venting and combustion air pathways for restrictions.

Resources And Standards

Reference materials include the furnace manufacturer’s service manual, the National Fuel Gas Code (NFPA 54/ANSI Z223.1), and local building codes. Utility companies and certified HVAC organizations offer guidance on safe gas appliance operation and certified contractor directories.

Key Takeaways

A pilot light that won’t stay lit is usually caused by a dirty pilot orifice, faulty thermocouple, drafts, gas supply problems, or a failing gas valve. Basic checks—observing flame, cleaning the orifice, repositioning or replacing the thermocouple, and ensuring proper gas flow—solve many issues. For gas leaks, complex control failures, or uncertain conditions, a licensed HVAC technician should be contacted.