Furnace Pilot Light but Burners Not Igniting: Causes and Fixes

The furnace pilot light is on but the main burners do not ignite, leaving a home without heat. This article explains common causes, step-by-step troubleshooting, safety precautions, repair options, and when to call a professional to restore reliable heating. The goal is to provide clear, actionable guidance for U.S. homeowners experiencing pilot-on/burners-off furnace issues.

Symptom	Likely Causes	Typical Fix
Pilot lit; burners won’t stay on	Faulty thermocouple/thermopile, gas valve issues	Replace thermocouple or call HVAC tech
Igniter or spark present but burners not lighting	Clogged burner ports, gas supply problem	Clean burners, check gas pressure, service by pro
Pilot out repeatedly	Drafts, dirty pilot orifice, weak thermocouple	Clean orifice, tighten connections, replace sensor

How A Furnace Pilot And Burner Ignition System Works

A traditional furnace uses a pilot light or electronic ignition to light the main burners. The pilot light is a small, continuously burning flame that heats a safety sensor such as a thermocouple or thermopile, which signals the gas valve to open. In modern systems, an electronic igniter or hot surface igniter provides the spark or heat instead of a continuously burning pilot.

When the thermostat calls for heat, the control system opens the gas valve and either the pilot or the igniter lights the main burners. If the pilot remains lit but the burners do not, the fault is typically in the safety sensor, gas valve control, or in the pathway between the valve and burners.

Common Reasons The Pilot Light Is On But Burners Won’t Light

1. Faulty Thermocouple Or Thermopile

The thermocouple is a safety device that senses the pilot flame and keeps the gas valve open. A thermopile produces more voltage for electronic controls. If either is weak, the control may not open the main gas valve even though the pilot remains lit. Symptoms include a steady pilot but no valve activation when the thermostat calls for heat.

2. Malfunctioning Gas Valve Or Solenoid

The gas valve or its solenoid controls gas flow to the burners. Electrical defects, internal failure, or stuck components can prevent gas from reaching burners. This is a critical part that often requires a licensed technician to diagnose and replace.

3. Dirty Or Clogged Burner Orifices

Over time, burners accumulate dust, rust, and spider webs that restrict gas flow or prevent flame propagation from the pilot to the main burner ports. Cleaning burner ports and ensuring proper alignment can restore ignition.

4. Faulty Thermostat Or Electric Control Board

If the thermostat does not send the call for heat signal or the control board fails to energize the gas valve or igniter, burners will not light. Verifying voltage at the valve during a heat call helps isolate this issue.

5. Airflow Or Draft Issues Around The Pilot

Strong drafts can distort the pilot flame or keep the pilot from properly heating the sensor. This may allow the pilot to burn yet not produce the consistent heat needed for the thermocouple to signal the valve.

6. Issues With Electronic Ignition Components

In furnaces with electronic ignition, components such as the hot surface igniter, flame sensor, or spark module can fail. The pilot or glow may be present but not sufficient to light the burners due to a weak igniter or dirty flame sensor.

Safety First: Precautions Before Troubleshooting

Before inspecting a furnace, turn off power at the service switch and shut off the gas if performing any maintenance that could release fuel. If a strong gas odor is present, leave the home immediately and call the gas utility or 911. Working on gas appliances can be hazardous; consider hiring a licensed HVAC technician for repairs involving the gas valve or internal furnace components.

Step-By-Step Troubleshooting Checklist

Tools And Supplies

• Multimeter for voltage checks
• Adjustable wrench and screwdriver set
• Small wire brush and compressed air
• Replacement thermocouple/thermopile (if needed)
• Manufacturer’s manual and parts diagram

1. Confirm Basic Conditions

Ensure the thermostat is set to heat and set above room temperature. Verify circuit breakers and furnace power switch are on. Check the gas supply valve to confirm it is open.

2. Observe Furnace Behavior During Heat Call

Set the thermostat to call for heat and watch the sequence. Note whether the control board clicks, whether the gas valve energizes, and whether any sparking or ignition attempt occurs. These observations narrow the fault to electrical control, gas supply, or ignition.

3. Test The Thermocouple Or Thermopile

With the pilot lit, measure the thermocouple/thermopile voltage. A thermocouple typically produces about 20-30 millivolts; a thermopile produces several hundred millivolts. Values below specification indicate replacement is needed. Also check that the sensor tip is properly positioned in the pilot flame.

4. Check For Voltage At The Gas Valve

During a heat call, measure voltage at the gas valve terminals. If the valve does not receive the correct signal voltage, the control board, thermostat wiring, or relay may be the problem. If the valve receives voltage but does not open, the valve is likely defective.

5. Inspect And Clean Burners

Turn off gas and power, then remove the burner assembly. Use a wire brush and compressed air to clear ports and openings. Reassemble, restore power, and test. Cleaning often resolves ignition problems caused by blockage.

6. Test Electronic Ignition Components

For electronic systems, inspect the hot surface igniter for cracks or discoloration and test flame sensor continuity. Replace a cracked igniter. Carefully clean the flame sensor with fine steel wool to remove oxidation, then retest operation.

When To Replace Parts Versus Call A Professional

Replacing a thermocouple or cleaning burners is often a homeowner-friendly task if comfortable working with small parts and following safety steps. Replacing a gas valve, control board, or performing detailed gas pressure diagnostics should be handled by a licensed HVAC technician due to safety, licensing, and code requirements.

If troubleshooting reveals no voltage to the gas valve, suspected refrigerant gas issues, or recurring pilot failures, contact a professional. A technician can perform leak checks, adjust gas pressure, and ensure compliant repairs.

Estimated Costs And Timeframes

Costs vary regionally. Typical ranges: thermocouple replacement $75–$150, burner cleaning $100–$200, igniter replacement $150–$350, gas valve replacement $300–$700, and full control board replacement $200–$600. Emergency service or complex repairs can increase costs.

Simple fixes like cleaning or thermocouple replacement often take under an hour. Gas valve or control board replacements may require 1–3 hours plus service diagnostics.

Preventive Maintenance To Avoid Future Problems

• Schedule annual HVAC tune-ups before the heating season to inspect pilots, burners, and sensors.
• Replace furnace filters regularly to reduce dust buildup that can reach burners and sensors.
• Keep the area around the furnace clear of clutter and avoid drafts near the pilot assembly.
• Consider upgrading to a modern electronic ignition system for improved reliability and efficiency.

Repair Walkthrough: Replacing A Thermocouple (Basic Guide)

This procedure suits homeowners comfortable with simple HVAC parts. Always follow the furnace manual and local codes.

1. Turn off furnace power and close the gas valve.
2. Locate the thermocouple mounting nut near the gas valve and pilot assembly.
3. Disconnect the thermocouple from the gas valve and remove the clip holding the sensor to the pilot assembly.
4. Install the new thermocouple, ensuring the sensor tip sits fully in the pilot flame, and tighten connections snugly.
5. Open the gas valve, restore power, relight the pilot per manufacturer instructions, and test operation.

Common Misdiagnoses And Pitfalls

Replacing the gas valve without testing the thermocouple is a frequent unnecessary expense. Similarly, failing to check electrical signals to the valve can lead to replacing parts that were not faulty. Always verify the call-for-heat sequence and measure voltages before replacing expensive components.

Cleaning the burner without addressing a weak thermopile may produce only temporary improvement. Take a systematic approach: observe, measure, then replace.

Frequently Asked Questions

Why Does The Pilot Stay Lit But The Burners Never Ignite?

Because the pilot only confirms limited flame presence. The main burners require a functioning sensor and gas valve activation. A weak thermocouple, failed gas valve, or control issue can permit pilot operation while preventing main burner ignition.

Can A Dirty Flame Sensor Prevent Main Burners From Lighting?

Yes. If the flame sensor does not register the pilot or igniter flame properly, the control system may block main gas flow as a safety measure.

Are Electronic Ignition Systems More Reliable Than Pilot Lights?

Electronic systems generally improve efficiency by eliminating a continuous pilot flame and reduce some failure points. However, their igniters and sensors can still fail and often require professional replacement.

Resources And Next Steps

Homeowners should consult the furnace owner’s manual for model-specific instructions and part numbers. For gas valve, pressure, or complex electrical issues, contact a licensed HVAC contractor. Keeping records of service, part replacements, and observed symptoms helps technicians diagnose problems more quickly.

Key Takeaway: A lit pilot with no main burners usually points to a safety sensor, gas valve, ignition component, or blocked burner ports. A methodical approach—observe, test, clean, and then replace—will identify the issue and guide safe repairs or professional service.