Furnace Igniter Glows but No Flame: Causes and Fixes

The furnace igniter glowing but no flame is a common heating issue that leaves many homeowners puzzled and cold. This article explains the most likely causes, step-by-step troubleshooting, safety considerations, expected repair costs, and preventive maintenance tips to restore reliable heat. Clear diagnostics and safe actions help determine when a DIY fix is appropriate and when to call a professional.

Summary Table

Symptom	Likely Causes	First Steps
Igniter glows, no ignition	Gas valve issue, faulty flame sensor, blocked burners	Check gas supply, clean sensor, inspect burners
Igniter cycles repeatedly	Control board or pressure switch problem	Observe error codes, reset power, check venting
Clicking, no spark/flame	Igniter failure, wiring fault	Visual inspection, continuity test

How Gas Furnace Ignition Works

Modern gas furnaces typically use a hot surface igniter or an intermittent pilot system to light the burners. The control board sends power to the igniter, which heats until it glows. When the igniter reaches operating temperature, the gas valve opens and the burners should ignite. A flame sensor confirms ignition and keeps the gas flowing.

Why The Igniter Glows But No Flame Occurs

When the igniter glows but there is no flame, the system is completing the first step but failing to progress. Common failure points include gas delivery, ignition sequencing, flame sensing, and airflow or safety interlocks. Identifying which subsystem is failing narrows the repair approach.

Common Causes Explained

1. Gas Supply Or Gas Valve Problems

If the igniter glows and the burners do not light, the gas valve may not be opening or fuel may not be available. Issues include closed valves, a tripped gas shutoff, or a failing gas control valve. A licensed technician should handle gas valve replacement.

2. Faulty Flame Sensor

The flame sensor detects a stable flame and signals the control board to keep the gas valve open. If the sensor is dirty or failing, the system may shut the gas off immediately after ignition, appearing as a glowing igniter with no lasting flame. Cleaning or replacing the sensor often resolves the issue.

3. Blocked Or Dirty Burners

Debris, dust, or spider webs in burner ports prevent proper gas flow and ignition. Burners that are misaligned or corroded also fail to light. Regular cleaning restores proper flame formation and combustion.

4. Ignition Control Board Or Sequencing Faults

The control board manages ignition timing, gas valve activation, and safety checks. A failed relay, timer, or sensor input can stop the sequence between igniter glow and gas release. Diagnostic LEDs or error codes on the board often point to control issues.

5. Pressure Switch Or Venting Issues

Combustion air and venting must be correct for safe ignition. A blocked vent, inducer motor failure, or stuck pressure switch can prevent the control board from allowing the gas valve to open, despite a glowing igniter.

6. Wiring And Electrical Connections

Loose or corroded wiring between the control board, igniter, flame sensor, and gas valve can interrupt signals. Intermittent connections can cause inconsistent behavior: the igniter may glow but the valve never receives the open command.

Safety First: What To Check Before Troubleshooting

Always prioritize safety by turning off power to the furnace at the breaker before opening panels. If the homeowner smells gas, evacuate and call the gas company or emergency services immediately. Only proceed with troubleshooting if no gas odor is present and the homeowner is comfortable performing basic checks.

Step-By-Step Troubleshooting Guide

Tools Needed

• Multimeter for continuity and voltage checks
• Screwdriver set and nut drivers
• Soft brush and fine-grit sandpaper or emery cloth
• Compressed air or vacuum with a brush attachment
• Manufacturer manual or model number for reference

1. Observe Furnace Behavior And Error Codes

Record what happens when the thermostat calls for heat. Note whether the inducer motor runs, if the igniter glows, and whether there are any flashing LED error codes on the control board. These observations guide the next steps.

2. Confirm Gas Supply

Verify other gas appliances work to rule out a supply interruption. Locate the furnace gas shutoff and ensure it is open. If any doubt exists about gas supply, contact the gas utility or a technician.

3. Inspect And Clean The Flame Sensor

Remove the flame sensor and gently clean the metal rod with fine-grit sandpaper or emery cloth to remove buildup. Reinstall securely. A dirty sensor is a common, fixable cause of ignition failure.

4. Check Burners And Igniter Position

Inspect burner ports for dust and obstructions. Clean with a brush or compressed air. Ensure the igniter is properly positioned and undamaged. Avoid touching ceramic parts of the igniter with bare hands.

5. Test The Gas Valve And Control Signals

With the thermostat calling for heat and the igniter glowing, use a multimeter to check for the control board output to the gas valve. If the board sends voltage to the valve but it does not open, the valve may be defective. If no voltage is present, the control board or sequencing is likely at fault.

6. Inspect Venting And Pressure Switch Operation

Check the flue and intake vents for blockages, birds nests, or snow. Verify the inducer motor runs. Test the pressure switch tubing for cracks or water. Replace faulty tubing and verify the pressure switch closes when the inducer creates the proper vacuum.

7. Examine Wiring And Connectors

Look for loose, burned, or corroded connectors. Wiggle harnesses while observing board behavior to identify intermittent faults. Repair or replace damaged wires and secure connections.

When Cleaning Or Simple fixes Might Work

Cleaning the flame sensor, burners, and clearing vents often solves the igniter-glows-but-no-flame problem. Tightening connectors and ensuring the gas valve is open are simple homeowner steps that can restore operation.

When To Call A Professional

Complex or hazardous issues—such as gas valve replacement, control board diagnosis, inducer motor repair, or persistent no-flame conditions after basic troubleshooting—require a licensed HVAC technician. Professionals have the tools to test gas pressures, handle gas components safely, and interpret control board diagnostics.

Typical Repair Actions And Estimated Costs

Costs vary by region and furnace model. The following ranges are typical for the U.S. and include labor and parts: flame sensor replacement $100–$250, igniter replacement $150–$400, gas valve replacement $300–$800, control board repair or replacement $250–$900, inducer motor replacement $300–$700. Injector cleaning or minor adjustments may be lower cost.

Parts Lifespan And When Replacement Is Advisable

Igniters and flame sensors wear over time. A hot surface igniter commonly lasts 3–10 years depending on cycles and model. Flame sensors may last many years but should be checked during annual service. Control boards and gas valves can fail unpredictably and are replaced as needed.

Preventive Maintenance To Avoid Ignition Problems

• Schedule annual professional furnace inspections before the heating season.
• Change air filters regularly to reduce dust entering the combustion chamber.
• Keep intake and exhaust vents clear of debris, snow, or obstructions.
• Clean flame sensors and burners during yearly tune-ups.
• Observe the furnace operation early in the season to catch minor issues before they fail in cold weather.

Safety Tips During Troubleshooting

Turn off electrical power before opening panels. Avoid handling gas components unless qualified. Do not spray flammable cleaners near the furnace. If gas odor is present, stop all work, ventilate, and call professionals or emergency services.

How To Read Furnace Error Codes

Many modern furnaces display diagnostic LED patterns on the control board. Consult the furnace manual or the label on the inside of the access panel to decode flash sequences. Error codes related to pressure switches, limit switches, or flame sensing narrow the fault to specific components.

Frequently Asked Questions

Q: Can a dirty flame sensor stop ignition even if the igniter glows?

A: Yes. A dirty sensor may not detect flame and will signal the board to shut off the gas, producing repeated cycles with the igniter glowing but no sustained flame.

Q: Is it safe to touch the igniter if it glows?

A: No. The igniter gets extremely hot and is brittle. Touching it can damage the element and cause burns. Power should be off before inspecting the igniter.

Q: Why does the furnace click but not ignite?

A: Clicking can indicate the control board trying to open the gas valve or a relay attempting to engage. If the valve does not open or the flame is not established, clicking can continue. Electrical or valve faults are common.

Q: Can a thermostat cause the igniter to glow but prevent gas flow?

A: Unlikely. A thermostat calls for heat and initiates the control sequence. If the igniter glows, the thermostat has completed its role. The issue is usually within the furnace components after that step.

Documentation And Model-Specific Guidance

Always consult the furnace manufacturer’s manual for model-specific wiring diagrams, diagnostic codes, and part numbers. Manufacturer documentation reduces guesswork and minimizes the risk of incorrect repairs or warranty voiding.

Final Diagnostic Checklist

1. Verify gas supply and open furnace gas shutoff.
2. Observe sequence: inducer, igniter glow, gas valve activation, burner ignition.
3. Clean the flame sensor and burners.
4. Inspect venting and pressure switches.
5. Test control board outputs and gas valve operation with a multimeter.
6. Secure or replace damaged wiring and connectors.
7. Call a licensed HVAC technician for gas valve, control board, or inducer motor issues.

Addressing a furnace where the igniter glows but the burners fail to ignite requires methodical troubleshooting and attention to safety. Simple cleaning and inspections often resolve the issue, while gas or control component failures demand professional service to ensure safe, reliable heating.