Quick answer
If your gas furnace burners won't stay lit, check the pilot light or ignition system, gas supply valve, and air shutter. Ensure the furnace is getting proper airflow and the flame sensor is clean.
Confirm the symptom
- Burners ignite briefly then go out
- Furnace makes clicking noise but no sustained flame
- Thermostat calls for heat but burners don't stay on
- Furnace cycles on and off rapidly
Tools and prep
- Screwdriver set
- Flashlight
- Gas leak detector or soapy water
- Safety glasses
- Turn off power to furnace at circuit breaker
- Ensure gas supply valve is open
Step-by-step troubleshooting
- Check Pilot Light or Ignition SystemDo: Locate the pilot light assembly and look for a steady blue flame. If using an electronic ignition, check for a spark.Observe: Pilot light is out or ignition system not sparkingMeans: Ignition system failure or gas supply issueNext: Relight pilot light if safe, or check ignition module
- Inspect Gas Supply ValveDo: Check the gas supply valve near the furnace to ensure it's fully open.Observe: Valve is closed or partially closedMeans: Gas flow is restrictedNext: Open valve fully and test furnace
- Clean Flame SensorDo: Locate the flame sensor near the burners and clean it with fine steel wool or sandpaper.Observe: Sensor is dirty or corrodedMeans: Flame sensor not detecting flameNext: Reinstall sensor and test furnace operation
- Check Air ShutterDo: Adjust the air shutter on the gas valve to ensure the correct air-to-gas mixture.Observe: Air shutter is too open or too closedMeans: Improper air mixture prevents stable flameNext: Adjust shutter and test burners
- Inspect Burner TubesDo: Remove burner tubes and clean them with a wire brush or compressed air.Observe: Burner tubes are clogged with debrisMeans: Blocked gas flow to burnersNext: Reinstall clean burners and test furnace
- Check for Gas LeaksDo: Apply soapy water to gas connections or use a gas leak detector.Observe: Bubbles form or detector alarmsMeans: Gas leak presentNext: Turn off gas supply and call a professional
Common causes
| Cause | Likelihood | Difficulty | Part often needed |
|---|---|---|---|
| Faulty Ignition System Ignition module or pilot light failure prevents sustained flame | Common | Medium | Ignition module or pilot assembly |
| Dirty Flame Sensor Flame sensor not detecting flame, causing furnace to shut off | Common | Easy | Flame sensor |
| Gas Supply Valve Closed Gas flow is restricted due to a closed valve | Sometimes | Easy | Gas valve |
| Clogged Burner Tubes Debris in burner tubes blocks gas flow | Sometimes | Easy–Medium | Burner tubes |
| Improper Air Shutter Adjustment Incorrect air-to-gas mixture prevents stable flame | Sometimes | Easy | Air shutter adjustment |
| Gas Leak Gas escaping before reaching burners | Rare | Hard | Gas line repair |
Parts matching
Common parts needed for a gas furnace with burners that won't stay lit:
- Ignition module or pilot assembly
- Flame sensor
- Gas valve
- Burner tubes
- Air shutter adjustment kit
Always match part specifications to your furnace model for compatibility.
When to call a professional
- Gas leak detected
- Furnace produces unusual noises or smells
- You're unsure about electrical or gas system repairs
- Multiple attempts to fix without success
- Error codes persist after troubleshooting
FAQ
Can a dirty flame sensor cause the burners to go out?
Yes, a dirty or corroded flame sensor can fail to detect the flame, causing the furnace to shut off for safety.
How do I know if my gas supply valve is open?
The valve handle should be parallel to the gas pipe when fully open. If it's perpendicular, it's closed.
Is it safe to relight the pilot light myself?
Yes, if the furnace is off and the area is well-ventilated. Follow the manufacturer's instructions carefully.
What should I do if I smell gas?
Immediately turn off the gas supply, evacuate the area, and call a professional or gas company.
Why is my furnace cycling on and off rapidly?
Rapid cycling can be caused by a dirty flame sensor, ignition issues, or improper air mixture.