Furnace Ignitor Lights but No Flame

The furnace ignitor glowing but no flame is a common heating issue that leaves homeowners cold and confused. This article explains likely causes, step-by-step troubleshooting, safety checks, repair options, and cost expectations to help identify and resolve the problem efficiently. Follow safety guidance before any inspection.

Symptom	Possible Causes	Quick Fixes
Ignitor Glows, No Burner Flame	Gas Supply Issue, Faulty Gas Valve, Dirty Flame Sensor, Blocked Burner, Pressure Switch Fault	Check gas valve, clean flame sensor, inspect vents, reset unit
Clicking Then Ignitor, No Ignition	Ignitor Failure, Control Board, Ignition Control Module	Replace ignitor, test for spark/continuity
Pilot Light On Older Units, Burner Won’t Light	Thermocouple Fault, Drafts, Clogged Orifice	Replace thermocouple, clean pilot assembly

How A Furnace Ignition System Works

Modern gas furnaces use an electronic ignitor or hot surface ignitor to light burners. The thermostat calls for heat and the furnace control board runs safety checks, opens the gas valve, and energizes the ignitor. Once the ignitor reaches operating temperature, gas enters the burners and the flame should establish. A flame sensor or thermocouple confirms flame and the system continues. Any interruption in that sequence prevents burners from sustaining a flame.

Common Causes When The Ignitor Lights But There Is No Flame

Gas Supply Problems

If the ignitor heats but burners remain unlit, the gas supply is a top suspect. This can be a closed gas valve, a tripped gas meter, or a utility interruption. A leaking or partially closed shutoff valve also reduces gas pressure below ignition threshold.

Faulty Gas Valve Or Valve Solenoid

The gas valve opens to feed burners after the ignitor warms. A defective valve or solenoid may not open despite the control board command. Symptoms include humming or clicking without gas flow. This requires component testing and often replacement.

Dirty Or Faulty Flame Sensor

When the burner lights then immediately shuts off, a dirty flame sensor can fail to detect flame, causing the control board to halt gas flow. However, if the ignitor lights and no flame appears, a dirty sensor may not be the primary cause, but cleaning the sensor is a low-cost, high-impact step.

Clogged Burners Or Orifice

Debris, rust, insect nests, or spider webs can block burner ports or orifices, preventing proper gas flow and mixing. Burners may glow or the ignitor may heat but gas cannot reach the flame pathway, producing no visible flame.

Pressure Switch Or Venting Issues

Modern furnaces use a pressure switch to confirm proper venting for combustion. If the switch detects a blockage or negative pressure anomaly, it will prevent gas valve operation. Blocked exhaust vents, flues, or inducer motor failures trigger this safety cutoff.

Control Board Or Ignition Module Failure

The control board coordinates ignition sequencing. If the board sends power to the ignitor but fails to open the gas valve, or if the ignition control module is faulty, burners will not ignite even though the ignitor works. Error codes on the control board can help diagnose these failures.

Thermocouple Or Pilot Issues On Older Furnaces

Older furnaces with standing pilot lights rely on a thermocouple. If the pilot is weak or the thermocouple fails, the gas valve will stay closed for safety. A healthy pilot flame and thermocouple continuity are required for operation.

Safety First: Pre-Checks Before Troubleshooting

Never bypass safety devices. Before investigating, turn off power to the furnace using the service switch and shut off the gas supply if a leak or strong smell of gas is suspected. Use a gas leak detector or soap test for visible fittings. If gas odor is present, leave the premises and call the gas company immediately.

Step-By-Step Troubleshooting Guide

1. Observe The Ignition Sequence

With the thermostat set to call for heat, watch the furnace: does the inducer motor start, does the ignitor glow, and then is there clicking or any gas odor? Note lights or error codes on the control board. Documenting the sequence narrows fault possibilities.

2. Confirm Gas Supply

Check the house gas meter and main valve. Ensure shutoff valves to the furnace are fully open. If multiple gas appliances are affected or meter is off, contact the utility. If other appliances work but the furnace does not, proceed to component checks.

3. Inspect And Clean Burners And Flame Sensor

Turn off power and gas. Remove burner assembly to check for dirt or blockages. Clean the flame sensor gently with fine-grit sandpaper or emery cloth. Reassemble and test. Cleaning the flame sensor often resolves intermittent ignition issues.

4. Check The Pressure Switch And Venting

Inspect induction blower operation and vent pipes for obstructions like bird nests or ice. A multimeter can test pressure switch continuity during starting sequence. If the inducer motor is noisy or fails, the pressure switch may not close.

5. Test The Gas Valve And Ignition Components

Using a multimeter, test for voltage to the gas valve when the ignitor operates. If voltage is present but valve does not open, the valve is likely defective. Test ignitor continuity and look for cracks if using a hot surface ignitor; replace if damaged.

6. Evaluate The Control Board

Check for error LED codes and consult the furnace manual for code interpretation. Control boards can fail to send the correct signals. Visual inspection may reveal burnt components or corrosion. Replacing the board can be costly but necessary if diagnostics point to it.

When To Call A Professional HVAC Technician

Call a licensed HVAC technician when the issue involves gas valve replacement, control board faults, inducer motor replacement, or when a gas leak or odor is present. Professionals have the tools to measure gas pressure, test components safely, and perform sealed combustion venting repairs.

DIY Repairs: What Homeowners Can Reasonably Do

• Clean the flame sensor and burner ports after shutting power and gas.
• Confirm gas shutoff valves are open and check other gas appliances.
• Reset the furnace by turning power off and on to clear electronic lockouts.
• Inspect vents and flue for visible blockages and remove debris outside the exhaust.

Do not attempt gas valve or control board replacement without proper training and local licensing where required.

Typical Costs And Timeframes

Costs vary by region and furnace type. Expect these national ranges: flame sensor cleaning or replacement $75–$200, ignitor replacement $150–$400, gas valve replacement $300–$700, control board replacement $300–$900, full inducer motor replacement $250–$700. Emergency service fees can add to costs. Many repairs are completed in 1–3 hours; complex control or gas valve jobs may take longer.

Preventive Maintenance To Avoid Future Ignition Failures

Schedule annual furnace tune-ups before heating season. A technician should clean burners, check flame sensor and thermocouple, test gas pressure, inspect the heat exchanger, and verify venting and inducer operation. Replace air filters every 1–3 months to maintain airflow and component longevity.

Parts And Tools Commonly Used In Repairs

Part	Purpose	Typical Lifespan
Hot Surface Ignitor	Provides heat to ignite gas	3–10 years
Flame Sensor	Detects flame presence for safety	5–10 years
Gas Valve	Controls gas flow to burners	10–20 years
Pressure Switch	Verifies proper venting and inducer operation	Varies

Common Diagnostic Codes And What They Mean

Many furnaces flash LED codes indicating issues. Typical codes: one blink may indicate a call for heat, multiple blinks often denote pressure switch failure, flame failure, or limit switch trip. Always consult the furnace’s specific manual. Documenting blink patterns helps technicians arrive prepared.

Safety Reminders And Regulatory Notes

Never disable safety devices like flame sensors, pressure switches, or limit switches. Working on gas appliances may require a licensed contractor per local codes. Carbon monoxide detectors should be installed and tested regularly in homes with combustion appliances.

Final Troubleshooting Checklist

1. Check for gas odor and shut off gas if detected.
2. Observe the ignition sequence and note error codes.
3. Verify gas supply and shutoff valves are open.
4. Clean flame sensor and burners.
5. Inspect venting and inducer motor operation.
6. Test voltage to gas valve and continuity of ignitor/thermocouple.
7. Call a licensed technician for gas valve, control board, or inducer replacements.

Addressing a furnace where the ignitor lights but no flame appears requires systematic diagnostics focused on gas delivery, safety interlocks, and ignition components. Many issues are preventable with regular maintenance; when in doubt, contact a qualified HVAC professional to ensure safe and effective repairs.