Why the Furnace Pilot Light Won’t Light and How to Fix It

The pilot light on a gas furnace serves as the ignition source for the burner, and when the pilot is not lighting it can leave a home without heat and raise safety concerns. This article explains common causes, troubleshooting steps, safety precautions, and when to call a professional. It focuses on natural gas and propane furnaces in the U.S. and provides actionable guidance for homeowners and renters.

Problem	Likely Cause	Quick Fix
Pilot Won’t Light	Dirty pilot orifice, clogged thermocouple, or empty gas supply	Clean pilot area, relight per manual, check gas supply
Pilot Lights But Won’t Stay Lit	Faulty thermocouple or thermopile, faulty pilot control valve	Replace thermocouple/thermopile or pilot assembly
No Gas To Pilot	Main gas valve off or gas supply interruption	Turn on gas valve, contact gas provider
Strong Odor Or Hissing	Gas leak	Evacuate, call gas company and 911 if necessary

How Furnace Pilot Systems Work

A pilot light is a small, continuously burning flame that ignites the furnace burner when heat is needed. Modern furnaces use one of two systems: a standing pilot with a thermocouple or thermopile that senses the flame, or an electronic ignition system that uses a hot surface igniter or spark igniter instead of a continuous pilot. Understanding which system is present helps pinpoint troubleshooting steps.

Common Causes When The Pilot Won’t Light

When a pilot will not light, issues typically fall into mechanical, electrical, or fuel-supply categories. Mechanical problems include clogged pilot orifice or damaged pilot tube. Electrical issues may involve a failed safety sensor such as a thermocouple. Fuel-supply causes include a closed gas shutoff valve or a gas outage. Environmental factors like drafts or debris can also prevent a pilot from establishing a reliable flame.

Troubleshooting Steps Before Calling A Technician

1. Confirm Safety First

Smell for gas or hear a hissing sound before attempting anything. If a gas odor or leak is suspected, evacuate immediately, avoid open flames and electrical switches, and call the gas company or 911. Do not attempt repairs if a leak is present.

2. Check Fuel Supply And Valves

Ensure the gas shutoff valve to the furnace is fully open. Verify that other gas appliances are working to confirm a supply. For propane systems, check the tank level. A pilot won’t light if the fuel source is off or empty.

3. Follow The Manufacturer’s Pilot Lighting Procedure

Every furnace has a specific relighting sequence printed on the furnace door or in the manual. This often includes turning the gas control to “pilot,” depressing a reset button, holding a knob down for 30–60 seconds while lighting, then releasing to see if the pilot stays lit. Following the proper timing is essential for the thermocouple to heat up.

4. Inspect The Pilot Flame

A healthy pilot flame is strong, mostly blue, and envelops the thermocouple. A weak, yellow, or flickering flame suggests debris, draft, or improper gas pressure. A pilot flame that does not contact the thermocouple will not allow the control valve to open for burner operation.

5. Clean The Pilot Orifice And Tube

Dust, spider webs, and soot can clog the pilot orifice. Turn off gas and power, then use compressed air, a fine wire, or a soft brush to clear the orifice and pilot tube. Reassemble and attempt relighting per the manual.

6. Test And Replace The Thermocouple Or Thermopile

If the pilot lights but goes out when the control knob is released, the thermocouple or thermopile likely fails. A multimeter can test output millivolts; a thermocouple typically produces around 25–30 mV and a thermopile higher. Replacement is a common homeowner repair for those comfortable with basic tools, but the gas supply must be turned off first.

7. Check For Drafts And Ventilation Problems

A strong draft near the pilot assembly can extinguish the flame. Check for open windows, fans, or exhaust vents blowing back into the furnace area. Adjusting drafts or repositioning the pilot shield may be necessary to maintain a stable flame.

When The Pilot Lights But The Furnace Won’t Run

If the pilot stays lit but the furnace burners will not ignite, suspect the main gas valve, control board, or limit switches. Thermostat wiring or settings can also prevent the burner from firing. Verify thermostat calls for heat and check any safety switches on the furnace, such as the pressure switch or limit switch, which can stop ignition if they sense unsafe conditions.

Safety Devices And Why They Prevent Pilot Ignition

Modern furnaces include multiple safety devices to prevent unsafe operation. The thermocouple senses pilot flame; the flame rollout switch detects improper combustion or blocked venting; the pressure switch ensures the inducer motor creates proper draft. If any of these fail or detect a problem, the furnace will not allow ignition. Diagnosing which safety device is preventing ignition often requires a technician with diagnostic tools.

How To Test Key Components

Thermocouple/ Thermopile Testing

With the pilot lit, disconnect the thermocouple from the gas valve and measure millivoltage across its terminals with a multimeter. Values significantly below manufacturer specifications indicate replacement. For thermopiles powering a gas valve directly, measure higher millivoltage and compare to specifications.

Gas Valve And Solenoid Checks

A faulty gas valve can prevent gas from reaching the pilot or main burners. Listen for a click when the thermostat calls for heat, indicating solenoid action. Testing or replacing a gas valve usually requires a professional due to safety regulations and the complexity of gas controls.

Ignition Control Board Diagnostics

Control boards often have LED codes that indicate fault conditions. Refer to the furnace manual for diagnostic sequences. Fault codes can identify issues such as flame failure, pressure switch faults, or inducer motor problems.

Common Repairs And Estimated Costs

Repair costs vary by region and furnace type. Typical straightforward repairs include pilor orifice cleaning (usually DIY or minimal cost), thermocouple replacement ($50–$150 for parts and labor), and thermopile replacement ($100–$300). More complex repairs like gas valve replacement or control board replacement can range from $300 to $900 or more. Always obtain multiple quotes from licensed HVAC professionals.

When To Call A Professional

Call a licensed HVAC technician if a gas leak is suspected, if replacement of gas valves or control boards is required, or if troubleshooting steps do not restore reliable operation. Professionals can safely test gas pressures, venting, combustion quality, and electrical controls. Hiring certified technicians protects safety and ensures compliance with local codes.

Preventive Maintenance To Avoid Pilot Problems

• Annual Furnace Tune-Up: Schedule professional service annually to clean burners, inspect the pilot orifice, check thermocouple function, and verify safe operation.
• Keep The Area Clean: Keep the furnace cabinet area free of dust, debris, and insects that can clog pilot orifices.
• Check Venting: Ensure flue and intake vents are clear of obstructions like bird nests or snow.
• Monitor Fuel Supply: Check propane tank levels and ensure gas meter and supply lines are functioning.
• Replace Old Components: Replace aging thermocouples, thermopiles, and control modules before they fail completely.

Special Considerations For Older Furnaces And Rentals

Older furnaces are more likely to use standing pilot systems, which require continual maintenance. For renters, contact the landlord or property manager for furnace issues rather than attempting extensive repairs. Building codes and landlord responsibilities often require licensed technicians for gas appliance work in rental properties.

Regulatory And Safety Guidance In The U.S.

Gas appliance repairs and installations are regulated at state and local levels. Many jurisdictions require licensed HVAC or plumbing contractors to perform gas valve or major fuel system repairs. Homeowners should consult local codes and utility companies before performing gas-related repairs. Utility companies often provide safety guidance and emergency response for suspected leaks.

Quick Checklist: Pilot Not Lighting

1. Smell For Gas—If Present, Evacuate And Call Emergency Services.
2. Verify Gas Supply And Open Gas Valves.
3. Follow Manufacturer Pilot Relight Procedure Exactly.
4. Inspect And Clean Pilot Orifice And Tube.
5. Check For Drafts Or Vent Obstructions.
6. Test Thermocouple/Thermopile Output With A Multimeter.
7. Inspect Safety Switches And Inducer Operation.
8. Call A Licensed Technician For Gas Valve, Control Board, Or Complex Issues.

Resources And Further Reading

Trusted sources for homeowners include the U.S. Consumer Product Safety Commission for appliance safety, local gas utility safety pages for emergency procedures, and manufacturer manuals for model-specific troubleshooting steps. HVAC trade associations and certified contractor directories can help find qualified technicians.

For additional guidance, consult the furnace owner’s manual, contact the local gas provider, or schedule a certified HVAC inspection when safety or complex repairs are involved.