Furnace Not Staying Lit

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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 stay lit, check the thermostat settings, ensure the pilot light is lit, and inspect the gas supply. If the pilot light goes out, relight it carefully or call a professional.

Confirm the symptom

Furnace ignites but shuts off shortly after
Pilot light stays lit briefly then goes out
Thermostat calls for heat but furnace cycles off quickly

Tools and prep

Screwdriver set
Flashlight
Gas leak detector
Safety glasses
Turn off furnace power at the circuit breaker
Ensure good ventilation in the area

Step-by-step troubleshooting

Check Thermostat Settings
Do: Verify the thermostat is set to 'Heat' and the desired temperature is higher than the current room temperature.
Observe: Thermostat is set correctly.
Means: Thermostat settings are not the issue.
Next: Check pilot light or ignition system
Check Pilot Light or Ignition System
Do: Look at the pilot light or electronic ignition system to see if it's lit. If not, follow manufacturer instructions to relight it.
Observe: Pilot light is lit or ignition system is functioning.
Means: Ignition system is operational.
Next: Inspect gas supply
Inspect Gas Supply
Do: Check the gas valve to ensure it's fully open. Look for any gas leaks using a gas leak detector.
Observe: Gas valve is open and no leaks detected.
Means: Gas supply is adequate and safe.
Next: Check flame sensor
Check Flame Sensor
Do: Locate the flame sensor near the burner and inspect it for dirt or corrosion. Clean it with a soft cloth or sandpaper if necessary.
Observe: Flame sensor is clean and making good contact.
Means: Flame sensor is likely not the issue.
Next: Test pressure switch
Test Pressure Switch
Do: Check the pressure switch for proper operation. A faulty switch can prevent the furnace from staying lit.
Observe: Pressure switch is functioning correctly.
Means: Pressure switch is not the issue.
Next: Call a professional
Call a Professional
Do: If the issue persists after these steps, contact a licensed HVAC technician.
Observe: Issue is resolved by a professional.
Means: Problem is beyond basic troubleshooting.
Next: None

Common causes

Cause	Likelihood	Difficulty	Part often needed
Dirty or faulty flame sensor A dirty flame sensor can't detect the flame, causing the furnace to shut off.	Common	Easy–Medium	Flame sensor cleaning kit
Blocked gas supply A partially closed gas valve or a gas leak can prevent proper fuel delivery.	Sometimes	Easy	Gas valve
Faulty ignition system A malfunctioning pilot light or electronic ignition can fail to keep the furnace lit.	Sometimes	Medium	Ignition module or pilot light assembly
Malfunctioning pressure switch A faulty pressure switch can prevent the furnace from staying lit due to incorrect pressure readings.	Sometimes	Medium	Pressure switch
Thermostat issues Incorrect thermostat settings or a faulty thermostat can cause the furnace to cycle improperly.	Rare	Easy	Thermostat
Electrical issues Loose or damaged wiring can interrupt the furnace's operation.	Rare	Medium	Wiring and connectors
Airflow blockage Blocked vents or a dirty air filter can cause the furnace to shut off for safety.	Rare	Easy	Air filter

Parts matching

If you need replacement parts for your furnace, consider these common items:

Flame sensor cleaning kit
Gas valve
Ignition module or pilot light assembly
Pressure switch
Thermostat
Air filter

Always match parts to your furnace model for compatibility.

When to call a professional

Furnace still won't stay lit after checking all basic issues
You detect a gas leak
You're unsure about any electrical or gas-related repairs
The furnace is old or has not been serviced recently
You hear unusual noises or smells during operation

FAQ

How do I know if my flame sensor is dirty?

A dirty flame sensor may cause the furnace to shut off shortly after igniting. You can inspect it visually for dirt or corrosion.

Can a bad thermostat cause my furnace to not stay lit?

A faulty thermostat can cause improper cycling, but it's less likely to be the main cause if the furnace ignites but then shuts off.

Is it safe to relight the pilot light myself?

Yes, if you follow the manufacturer's instructions carefully. Ensure the area is well-ventilated and there are no gas leaks before attempting.

How often should I replace my air filter?

Replace your air filter every 1–3 months, depending on usage and filter type, to maintain proper airflow and furnace efficiency.

What should I do if I smell gas?

Immediately shut off the gas supply, ventilate the area, and call a professional. Do not attempt to relight the furnace or use any electrical devices.