Furnace Not Lighting Flame

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Quick answer
Confirm the symptom
Tools and prep
Step-by-step troubleshooting
Common causes table
Parts matching
When to call a pro
FAQ
Related guides

Quick answer

If your furnace won't light a flame, first check the thermostat settings and power. Then verify the pilot light is on. If it's out, relight it carefully. If the furnace still won't light, the gas valve or igniter may need attention.

Confirm the symptom

Furnace fan runs but no heat
No clicking or ignition sound when furnace starts
Pilot light is out or won't stay lit
Thermostat is set to heat but furnace doesn't respond

Tools and prep

Flashlight
Screwdriver
Gas leak detector or soapy water
Thermometer (optional)
Turn off furnace power at the thermostat or circuit breaker
Ensure gas supply is on

Step-by-step troubleshooting

Check Thermostat Settings
Do: Verify the thermostat is set to 'Heat' and the temperature is above room temperature.
Observe: Thermostat settings are correct.
Means: Thermostat is functioning properly.
Next: Check power supply
Check Power Supply
Do: Ensure the furnace power is on at the circuit breaker and the emergency switch on the furnace is in the 'On' position.
Observe: Power is restored and the furnace control panel lights up.
Means: Power supply is working.
Next: Check pilot light
Check Pilot Light
Do: Look through the access panel to see if the pilot light is lit. If it's out, follow the manufacturer's instructions to relight it.
Observe: Pilot light is lit and stable.
Means: Pilot light is functioning.
Next: Check gas supply
Check Gas Supply
Do: Verify the gas valve near the furnace is fully open. Use soapy water to check for gas leaks around the connections.
Observe: Gas valve is open and no bubbles appear in the soapy water.
Means: Gas supply is intact and safe.
Next: Test Ignition System
Test Ignition System
Do: Listen for a clicking sound when the thermostat calls for heat. If you hear clicking but no flame, the igniter or flame sensor may be faulty.
Observe: Clicking sound is present but no flame is produced.
Means: Ignition system may be malfunctioning.
Next: Inspect Flame Sensor
Inspect Flame Sensor
Do: Turn off the furnace power. Locate the flame sensor and clean it with a soft cloth or fine sandpaper to remove soot or debris.
Observe: Flame sensor is clean and free of debris.
Means: Flame sensor is likely functioning properly.
Next: Restart Furnace
Restart Furnace
Do: Restore power to the furnace and set the thermostat to call for heat. Observe if the furnace ignites properly.
Observe: Furnace ignites and produces heat.
Means: Issue is resolved.
Next: Monitor operation

Common causes

Cause	Likelihood	Difficulty	Part often needed
Pilot Light Out The pilot light can go out due to drafts, thermocouple issues, or gas supply problems.	Common	Easy	Thermocouple
Gas Supply Issues A closed gas valve or gas line blockage can prevent the furnace from lighting.	Sometimes	Easy	Gas valve
Faulty Igniter A worn or cracked igniter may fail to produce a spark or heat to ignite the gas.	Sometimes	Medium	Igniter
Dirty Flame Sensor A dirty flame sensor can prevent the furnace from detecting a flame and shutting off the gas.	Sometimes	Easy	Flame sensor cleaning kit
Thermostat Problems Incorrect settings or a faulty thermostat can prevent the furnace from receiving a signal to heat.	Sometimes	Easy	Thermostat
Electrical Issues Tripped circuit breakers or wiring problems can cut power to the furnace.	Rare	Medium	Electrical components
Blocked Ventilation Blocked vents or a dirty air filter can restrict airflow and prevent proper ignition.	Rare	Easy	Air filter

Parts matching

Common parts needed for a furnace not lighting flame include:

Thermocouple
Gas valve
Igniter
Flame sensor cleaning kit
Thermostat
Air filter

Before purchasing parts, ensure the furnace model is compatible and consider professional installation for complex components.

When to call a professional

You smell gas or suspect a gas leak
You hear a clicking sound but no flame is produced after multiple attempts
The pilot light won't stay lit after relighting
You're uncomfortable working with gas or electrical components
The furnace is over 15 years old and frequent repairs are needed

FAQ

How do I know if my pilot light is out?

Look through the access panel on the furnace. If the small blue flame is not visible, the pilot light is out. You may also notice the furnace not producing heat despite the thermostat being set to 'Heat'.

Can a dirty air filter cause the furnace not to light?

Yes, a very dirty air filter can restrict airflow, leading to improper combustion and preventing the furnace from lighting. Replace or clean the air filter regularly.

Why does my pilot light keep going out?

A pilot light that keeps going out may be due to a faulty thermocouple, drafts, or gas supply issues. Check the thermocouple alignment and gas valve settings.

Is it safe to relight the pilot light myself?

Yes, if you follow the manufacturer's instructions carefully. Ensure the gas valve is closed before attempting to relight and never use open flames near the furnace.