Furnace Not Lighting

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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

Check the thermostat settings, ensure the gas valve is open, and inspect the pilot light or ignition system. If the furnace still won't light, the issue may be with the thermocouple, gas pressure, or a faulty ignition module.

Confirm the symptom

No heat output from the furnace
No clicking or ignition sound when turning on the thermostat
Furnace fan runs but no burner lights
Thermostat shows no error but furnace doesn't respond
Pilot light is out (if applicable)

Tools and prep

Screwdrivers (flathead and Phillips)
Multimeter (for voltage testing)
Flashlight
Gas leak detector or soapy water
Safety gloves and goggles
Owner's manual for the furnace
Turn off the power at the circuit breaker
Ensure the gas supply is on and accessible

Step-by-step troubleshooting

Check Thermostat Settings
Do: Verify the thermostat is set to 'Heat' and the temperature is higher than the room temperature.
Observe: Thermostat is set correctly but furnace still doesn't light.
Means: Thermostat settings are not the issue.
Next: Check the gas supply and pilot light.
Inspect Gas Supply
Do: Locate the gas valve near the furnace and ensure it's fully open.
Observe: Gas valve is closed or partially closed.
Means: The furnace isn't receiving gas.
Next: Open the gas valve and attempt to relight the pilot light.
Check Pilot Light
Do: Look through the access panel to see if the pilot light is lit.
Observe: Pilot light is out.
Means: The pilot light needs to be relit.
Next: Relight the pilot light following the manufacturer's instructions.
Relight the Pilot Light
Do: Follow the furnace's manual instructions to manually light the pilot light.
Observe: Pilot light stays lit but the furnace still doesn't ignite.
Means: The thermocouple may be faulty.
Next: Test the thermocouple with a multimeter.
Test Thermocouple
Do: Use a multimeter to check the thermocouple's voltage output when the pilot light is lit.
Observe: Voltage is below 250 mV.
Means: The thermocouple is not generating enough voltage to keep the gas valve open.
Next: Replace the thermocouple.
Check Ignition System
Do: Inspect the ignition module and spark electrode for signs of wear or damage.
Observe: Spark electrode is dirty or damaged.
Means: The ignition system is not functioning properly.
Next: Clean or replace the spark electrode.
Inspect Gas Pressure
Do: Use a gas pressure gauge to check the gas pressure at the furnace inlet.
Observe: Gas pressure is too low.
Means: The gas pressure regulator may be faulty.
Next: Contact a professional HVAC technician to adjust or replace the regulator.
Check for Gas Leaks
Do: Apply soapy water to gas connections and look for bubbles, or use a gas leak detector.
Observe: Bubbles appear at a connection point.
Means: There is a gas leak that needs to be addressed immediately.
Next: Shut off the gas supply and contact a professional.

Common causes

Cause	Likelihood	Difficulty	Part often needed
Closed Gas Valve The furnace isn't receiving gas, preventing the pilot light or burners from igniting.	Common	Easy	N/A
Out Pilot Light A pilot light that's gone out will prevent the furnace from igniting.	Common	Easy	N/A
Faulty Thermocouple The thermocouple generates a small voltage to keep the gas valve open; if it's faulty, the gas will shut off.	Common	Easy–Medium	Thermocouple
Dirty or Faulty Ignition System A dirty or damaged spark electrode can prevent the ignition process.	Sometimes	Easy–Medium	Ignition module or spark electrode
Low Gas Pressure Insufficient gas pressure can prevent the furnace from igniting properly.	Sometimes	Hard	Gas pressure regulator
Gas Leak A gas leak can prevent the furnace from lighting and poses a safety hazard.	Rare	Hard	Professional repair
Faulty Circuit Board A malfunctioning circuit board can prevent the furnace from receiving the signal to ignite.	Sometimes	Hard	Control board
Clogged Burners Dust or debris in the burners can block the gas flow and prevent ignition.	Sometimes	Medium	Burner cleaning kit

Parts matching

Common parts needed for a furnace not lighting issue include:

Thermocouple
Ignition module or spark electrode
Gas pressure regulator
Control board
Burner cleaning kit

Always refer to your furnace's model number when purchasing replacement parts to ensure compatibility.

When to call a professional

You detect a gas leak (smell of rotten eggs, bubbles from soapy water).
The furnace still doesn't light after replacing the thermocouple and cleaning the burners.
You're unsure about handling electrical or gas components.
The furnace is over 15 years old and frequent repairs are needed.
Error codes persist despite troubleshooting.

FAQ

Why won't my furnace light even after relighting the pilot?

If the pilot light goes out again or the furnace still doesn't ignite, the thermocouple may be faulty and need replacement.

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

A dirty air filter can restrict airflow and cause the furnace to shut down for safety, but it typically won't prevent the furnace from lighting initially.

Is it safe to relight the pilot light myself?

Yes, if you follow the manufacturer's instructions carefully. However, if you're unsure or smell gas, stop and call a professional immediately.

How often should I check my furnace's pilot light?

During seasonal maintenance, check the pilot light and overall furnace operation to ensure everything is working correctly.

What should I do if I smell gas near the furnace?

Immediately shut off the gas supply, ventilate the area, and contact a professional HVAC technician or gas company.