Furnace Pilot Light Is Lit but Burners Won’t Ignite: Causes and Solutions

When a furnace pilot light is lit but the burners won’t ignite, it can indicate an underlying issue that impacts heating efficiency and home comfort. This problem can result from several factors including faulty components, blocked parts, or safety mechanisms preventing ignition. Understanding the causes and troubleshooting steps can help homeowners quickly address the problem or decide when to call a professional.

Common Causes	Symptoms	Potential Solutions
Thermocouple failure	Pilot stays lit but burners do not ignite	Replace thermocouple
Clogged or dirty burners	Burners don’t light or flame is weak	Clean burners thoroughly
Faulty gas valve	No gas flow to burners despite pilot lit	Inspect and replace gas valve
Blocked or closed gas supply	Pilot lit but inadequate gas to burners	Open valves and check gas line
Control board or ignition system issues	Burner ignition failure despite pilot light	Service control board or ignition module

How a Furnace Pilot Light and Burner Ignition Work

A gas furnace relies on the pilot light to provide a small, constant flame that ignites the main burners when the thermostat signals for heat. The pilot light heats the thermocouple sensor, which then signals the gas valve to open and deliver fuel to the burners. Without this signal, burners won’t ignite even if the pilot flame is present.

Understanding this sequence is critical for diagnosing why burners don’t ignite while the pilot light remains lit.

Common Causes Why Burners Won’t Ignite When Pilot Light Is On

Thermocouple Failure or Malfunction

The thermocouple acts as a safety device that detects the pilot flame and signals the gas valve to open for the main burners. If it fails or its connection weakens, the gas valve won’t open. Though the pilot flame remains lit, the burners will not ignite because they aren’t receiving gas.

Thermocouple wear, dirt, or misalignment can cause this issue.

Clogged or Dirty Burners

Burners clogged with dust, soot, or debris restrict gas flow or create uneven flame distribution, causing ignition failure. Even with the pilot light active, inadequate gas delivery at the burner ports prevents ignition. Regular cleaning can alleviate this problem.

Faulty Gas Valve

The gas valve controls fuel flow to both the pilot and main burners. Sometimes the valve remains open for the pilot flame but fails to send gas to the main burners. This could be due to valve mechanical failure or electrical issues in the control system.

Blocked or Closed Gas Supply

Partial blockage, closed shutoff valves, or gas line issues can restrict the fuel flow to the burners. While the pilot light is a smaller flame and may still be lit, the larger burner flames require more gas. Any obstruction or restriction prevents proper burner ignition.

Ignition System or Control Board Problems

Modern furnaces often rely on electronic ignition systems and control boards rather than standing pilot lights. Faults in ignition modules, pressure switches, or the control board itself can cause ignition failure. Even if the pilot light is artificially kept lit, burners won’t ignite without proper control signals.

Troubleshooting Steps for Furnace Pilot Light Lit but Burners Won’t Ignite

1. Inspect the Thermocouple

Check if the thermocouple is properly positioned in the pilot flame. A misaligned or dirty thermocouple can fail to detect the flame. Cleaning it gently or tightening connections may help. If damaged, it requires replacement.

2. Clean the Burners

Turn off the furnace completely and clean the burners with a brush or compressed air. Remove debris or soot build-up to ensure proper gas flow and ignition.

3. Verify Gas Supply and Valves

Ensure all gas valves leading to the furnace are fully open. Confirm the gas line is unobstructed and the gas meter indicates normal service.

4. Test the Gas Valve Function

If a multimeter is available, verify that the gas valve receives proper voltage and opens when signaled. Faulty valves typically require professional replacement.

5. Evaluate the Ignition System

For furnaces using electronic ignition, observe error codes on the control board (if present) and reference the manual. Malfunctions in control components may require skilled repair or replacement.

When to Call a Professional HVAC Technician

If basic troubleshooting doesn’t resolve the issue, or if the problem involves gas valve replacement, control board diagnosis, or complex electrical systems, calling a licensed HVAC technician is recommended. Gas furnace repairs require expertise to ensure safety and code compliance.

Preventive Maintenance Tips to Avoid Ignition Issues

• Schedule annual furnace inspections and tune-ups by certified technicians.
• Clean burners and pilot assembly regularly to prevent soot buildup.
• Check and replace worn thermocouple sensors promptly.
• Keep gas lines and valves clear and fully open to ensure steady gas flow.
• Test ignition components and control systems during routine service visits.

Summary Table: Diagnosing Furnace Burners Not Igniting with Pilot Light On

Cause	Effect	Troubleshooting	Solution
Thermocouple Fault	Gas valve does not open for burners	Check flame contact and sensor connection	Clean, realign, or replace thermocouple
Dirty Burners	Burner flame weak or no ignition	Inspect burners for debris and blockage	Clean burners thoroughly
Gas Valve Problem	Gas does not reach main burners	Test valve operation and electrical signals	Replace valve if faulty
Gas Supply Issues	Insufficient gas flow to burners	Check valve positions and gas meter	Clear blockage and open valves
Ignition or Control Fault	Burners fail to ignite despite pilot	Review error codes and test components	Service/replace ignition or control module