Quick answer
If your furnace pilot light is on but it won't fire up, check the gas valve is fully open, ensure the thermocouple is properly positioned in the flame, and verify the gas pressure is correct. If these are fine, the ignition system or gas valve may need professional attention.
Confirm the symptom
- Pilot light is lit and steady
- Furnace does not ignite when thermostat calls for heat
- No clicking or burner ignition sounds
- Thermostat is set to heat and functioning
- Gas supply is on and working
Tools and prep
- Screwdriver (flathead and Phillips)
- Flashlight
- Gas leak detector or soapy water solution
- Thermometer (optional, to check gas pressure)
- Safety goggles
- Turn off power to furnace at circuit breaker
- Ensure gas supply is on but not over-pressured
Step-by-step troubleshooting
- Check Gas Valve PositionDo: Locate the gas valve near the pilot light and ensure it is fully open.Observe: Gas valve handle should be parallel to the gas line.Means: A closed or partially closed valve restricts gas flow.Next: If valve is closed, open it and wait 5 minutes before attempting to relight the pilot light.
- Inspect ThermocoupleDo: Use a screwdriver to gently adjust the thermocouple so it sits in the pilot light flame.Observe: Thermocouple should be positioned so the end is in the blue cone of the flame.Means: Improper positioning prevents the thermocouple from generating enough voltage to keep the gas valve open.Next: If the thermocouple is damaged or corroded, it may need replacement.
- Test Gas PressureDo: If you have a gas pressure gauge, check the pressure at the gas valve.Observe: Pressure should be within the manufacturer's specifications (usually 3.5–5 inches of water column).Means: Low gas pressure can prevent the burners from igniting.Next: If pressure is low, check for gas supply issues or consult a professional.
- Check Ignition SystemDo: Listen for a clicking sound when the thermostat calls for heat. If there's no sound, check the ignition control module.Observe: No clicking may indicate a faulty ignition system.Means: The ignition system may be defective or not receiving power.Next: If no clicking, turn off power and consult a professional.
- Inspect Burner TubesDo: Visually inspect the burner tubes for dirt, debris, or rust.Observe: Clean, unobstructed burner tubes are necessary for proper gas flow.Means: Blocked burners prevent gas from reaching the ignition point.Next: Clean the burners with a soft brush or compressed air if needed.
- Verify Power SupplyDo: Check the circuit breaker for the furnace to ensure it is on and not tripped.Observe: Breaker should be in the 'on' position and not warm or damaged.Means: A tripped breaker interrupts power to the furnace components.Next: Reset the breaker if tripped and test the furnace again.
Common causes
| Cause | Likelihood | Difficulty | Part often needed |
|---|---|---|---|
| Gas Valve Not Fully Open The gas valve must be fully open to allow sufficient gas flow to the burners. | Common | Easy | Gas Valve Handle |
| Improper Thermocouple Position The thermocouple must be positioned in the pilot light flame to generate the voltage needed to keep the gas valve open. | Common | Easy | Thermocouple |
| Low Gas Pressure Insufficient gas pressure prevents the burners from receiving enough fuel to ignite. | Sometimes | Medium | Gas Pressure Regulator |
| Faulty Ignition System A defective ignition control module or failed spark igniter can prevent the burners from igniting. | Sometimes | Medium–Hard | Ignition Control Module |
| Clogged Burner Tubes Dirt, dust, or rust in the burner tubes can block gas flow to the burners. | Sometimes | Easy–Medium | Burner Tubes or Cleaning Kit |
| Tripped Circuit Breaker A tripped breaker cuts power to the furnace, preventing the ignition system from functioning. | Rare | Easy | Circuit Breaker |
Parts matching
Common parts needed to resolve a furnace that won't fire up despite a lit pilot light include:
- Gas Valve Handle – Adjust or replace if the valve is not fully open.
- Thermocouple – Replace if damaged or improperly positioned.
- Gas Pressure Regulator – Adjust or replace if gas pressure is too low.
- Ignition Control Module – Replace if the ignition system is faulty.
- Burner Tubes – Clean or replace if clogged with debris.
- Circuit Breaker – Reset or replace if tripped or damaged.
Always ensure the gas supply is off before replacing or adjusting any parts. If unsure, consult a licensed HVAC technician.
When to call a professional
- If the pilot light goes out after adjustments
- If there's a gas smell after checking for leaks
- If the furnace still won't fire up after following all steps
- If you're uncomfortable working with gas or electrical components
- If the thermocouple or ignition system is damaged and needs replacement
FAQ
Why is my furnace pilot light on but it won't heat?
A lit pilot light but no heat usually indicates an issue with gas flow, thermocouple positioning, or the ignition system. Check the gas valve, thermocouple, and burner tubes first.
Can a dirty filter cause this issue?
A dirty air filter can restrict airflow and cause the furnace to shut down, but it typically won't prevent the burners from igniting if the pilot light is on. However, it's a good idea to check and replace the filter as part of routine maintenance.
Is it safe to relight the pilot light myself?
Yes, if you follow the manufacturer's instructions and ensure the gas supply is off before attempting. If you smell gas or are unsure, contact a professional immediately.
How often should I check my furnace's pilot light and thermocouple?
It's recommended to inspect your furnace annually, ideally before the heating season begins. Check the pilot light and thermocouple for proper positioning and cleanliness during routine maintenance.
What should I do if I smell gas?
Immediately shut off the gas supply, evacuate the area, and contact your gas company or emergency services. Do not attempt to relight the pilot light or use any electrical devices.