Furnace Pilot Light Lit but Burners Won’t Ignite: Troubleshooting and Fixes

The pilot light staying lit while the furnace burners won’t ignite is a common issue that can leave a home cold and homeowners frustrated. This guide explains likely causes, diagnostic steps, safety considerations, and practical fixes so technicians or handy homeowners can identify the problem and restore reliable heating. Safety first: follow all precautions and call a licensed HVAC professional for complex repairs or gas-related work.

Symptom	Possible Causes	Quick Fix
Pilot light on, burners not igniting	Faulty thermocouple, clogged orifice, bad gas valve, igniter failure, control board issue	Inspect thermocouple, clean burner area, check gas supply, test control components
Pilot flickers or goes out	Drafts, dirty pilot tube, weak thermocouple	Clear drafts, clean pilot tube, replace thermocouple

How A Standing Pilot System Works

A standing pilot system uses a small, continuously burning pilot flame to ignite the main burners when the thermostat calls for heat. The pilot flame heats a thermocouple or flame sensor that signals the gas valve to stay open. When the thermostat requests heat, the gas valve sends gas to the burners, where the pilot ignites the main flame. If any component in this chain fails, burners may not light even though the pilot stays lit.

Common Causes When Pilot Light Is Lit But Burners Won’t Ignite

Faulty Thermocouple Or Flame Sensor

The thermocouple (or flame sensor in newer units) detects the pilot flame and allows the gas valve to remain open. A weak, dirty, bent, or misaligned thermocouple may not generate sufficient voltage to operate the valve. Symptoms: pilot steady, main gas valve won’t open, furnace fails to light.

Defective Gas Control Valve

The gas control valve regulates fuel to both pilot and main burners. It may permit pilot flow but fail to open the main gas pathway due to mechanical failure, internal wear, or electrical faults. Valve issues often require professional replacement because of gas safety and code regulations.

Clogged Orifices Or Blocked Burner Ports

Dirt, rust, spider webs, and combustion residue can obstruct pilot tubes, burner orifices, or ports. A clear pilot doesn’t guarantee the main burner passages are clean. Restriction prevents proper gas flow and ignition.

Ignition System Malfunction (For Intermittent Or Electronic Ignition)

In modern furnaces with electronic ignition, the pilot is simulated by a hot surface igniter or spark. A stuck pilot position or faulty control may show a pilot flame indicator while the actual ignition sequence fails. Igniters, ignitor relays, and control modules are frequent culprits.

Faulty Thermostat Or Wiring Problems

A thermostat that doesn’t signal the furnace correctly or damaged wiring between the thermostat and control board prevents activation of the gas valve even when the pilot is burning. Electrical faults can interrupt the call for heat or the gas valve activation sequence.

Blocked Venting Or Pressure Switch Issues

High-efficiency furnaces rely on inducer motors and pressure switches to confirm safe venting. A clogged condensate drain, blocked vent, or failed pressure switch can disable the combustion sequence after the pilot stage, stopping burners from igniting.

Safety Precautions Before Troubleshooting

Working on gas appliances involves risk. Ensure the work area is well-ventilated and free of ignition sources. If there is a distinct gas odor, evacuate immediately and call the gas company or emergency services. Always shut off power and gas supply before inspecting components. Wear protective gloves and eyewear. When in doubt, contact a licensed HVAC technician.

Step-By-Step Troubleshooting Guide

1. Confirm Thermostat Call For Heat

Set the thermostat several degrees above room temperature and listen for the furnace to respond. If the fan turns on or the inducer motor starts, the thermostat is sending a signal. If no response, check thermostat batteries, settings, and wiring connections.

2. Observe The Furnace Sequence

Turn on the furnace and watch the ignition sequence. Note whether the pilot remains on, if the inducer motor starts, whether a click or gas flow is heard when the thermostat calls, and whether any error codes flash on the control board. These observations narrow potential failures.

3. Inspect And Test The Thermocouple Or Flame Sensor

Visually inspect the thermocouple for slope, corrosion, and pilot flame contact. Clean a flame sensor with a soft cloth or fine emery cloth to remove carbon buildup. Use a multimeter to test thermocouple millivolt output; a healthy thermocouple usually reads 25–30 mV when heated. Replace if readings are low or unstable.

4. Check The Gas Supply And Valve

Ensure the gas shutoff at the furnace and the meter is open. Listen for the gas valve actuating click when the thermostat calls. If the pilot is lit but there is no click or gas flow to the burners, the gas control valve may be defective or the control circuit may be failing.

5. Clean Burners And Pilot Tube

Turn off gas and power before cleaning. Remove burner assembly and inspect for spider webs, dust, or rust blocking ports. Clean with a soft brush or compressed air. Carefully clear pilot tube obstruction with a fine wire or compressed air. Reassemble and retest.

6. Inspect Igniter And Control Board (Electronic Ignition Systems)

Check the hot surface igniter for cracks or discoloration; test continuity with a multimeter. Verify control board outputs deliver voltage to the gas valve when calling for heat. Look for error LED patterns and consult the furnace manual to decode faults.

7. Test Pressure Switch And Venting Components

Ensure the inducer motor runs and the pressure switch closes during startup. Check condensate drains, PVC vent pipe, and external vent cap for blockages. A stuck-open or stuck-closed pressure switch can interrupt ignition even when the pilot is active.

When To Replace Parts

Replace a thermocouple, flame sensor, or igniter if cleaning and adjustment don’t restore reliable operation. Gas valves and control boards should be replaced only after confirming electrical inputs and outputs fail to operate, and replacement is performed by a qualified technician. Replacing parts can resolve intermittent failures and improve furnace safety.

Maintenance Tips To Prevent Future Problems

• Annual Professional Tune-Up: A licensed HVAC technician should perform yearly inspections, cleanings, and safety checks.
• Keep Vents Clear: Ensure return and exhaust vents remain unobstructed and outdoor vent caps are free of debris.
• Change Filters Regularly: Replace air filters every 1–3 months to reduce dust buildup that can affect burners and components.
• Seal Duct Leaks: Reduced airflow can stress the heating system and contribute to pressure switch errors.
• Keep Area Clean: Minimize dust, dirt, and insect ingress around the furnace to prevent orifice and pilot tube blockage.

Cost Considerations And When To Call A Professional

DIY fixes like cleaning the flame sensor or replacing a thermocouple are inexpensive and often successful. Costs for professional service vary regionally; expect routine diagnostics and minor repairs to range from modest service fees to a few hundred dollars. Major replacements like gas valves or control boards are costlier and require licensed professionals due to safety and code compliance.

Troubleshooting Checklist For Technicians

1. Verify thermostat call for heat and inspect wiring continuity.
2. Observe ignition sequence and note control board error codes.
3. Measure thermocouple millivolt output and sensor resistance.
4. Confirm gas supply and listen for gas valve actuation.
5. Inspect and clean burner ports, pilot tube, and flame sensor.
6. Test inducer motor operation and pressure switch functionality.
7. Test control board outputs to gas valve and igniter circuits.
8. Replace failed components and re-check for proper operation and combustion safety.

Diagnostic Table: Symptoms, Likely Causes, And Actions

Symptom	Likely Cause	Recommended Action
Pilot steady; no main flame	Thermocouple weak, gas valve stuck	Test thermocouple, clean/replace; check gas valve
Pilot flickers or goes out	Pilot tube obstruction, draft, weak thermocouple	Clean tube, adjust pilot flame, replace thermocouple
Inducer runs, no ignition	Pressure switch open, blocked vent, control board fault	Inspect venting, test pressure switch, read control codes
Click heard, no gas smell	Gas valve not opening internally, electrical fault	Measure valve coil voltage, replace valve if needed

Key Safety And Compliance Notes

Any work involving gas lines, gas valves, or major furnace components must adhere to local codes. Gas appliance servicing often requires licensed professionals to ensure safe operation and to maintain warranty coverage. Carbon monoxide risk increases with incomplete combustion; install and maintain CO detectors in the home.

Resources And Further Reading

For detailed procedures, refer to furnace manufacturer service manuals and NFPA (National Fire Protection Association) guidelines on gas appliance safety. Local utility companies and licensed HVAC professionals provide diagnostic services and advice for gas-related issues.

Following this guide helps diagnose why a pilot light may remain lit while main burners fail to ignite and outlines safe, effective steps to repair or direct professional service. When in doubt about gas handling or electrical testing, select a licensed technician to ensure safety and compliance.