Furnace Pilot Light Won't Stay Lit

By HVAC Load Calculate Team, licensed HVAC professionals · Updated July 2026

The most likely cause is failed or dirty thermocouple, or dirty pilot orifice blocking gas flow. Pilot light on gas furnace goes out repeatedly or won't stay lit after relighting. Most cases take diy relight: 15 minutes | thermocouple replacement: 1 hour | professional service: 1-2 hours to resolve once you know which cause you are dealing with, and the checks below start with the ones you can do yourself in a few minutes.

How to Confirm This Is Your Problem

You are probably dealing with this exact issue if you notice one or more of these signs:

Pilot light goes out after releasing button
No flame visible in pilot assembly
Furnace won't ignite or stay running
Gas smell when pilot won't light
Pilot flame is weak or yellow instead of blue
Pilot stays lit briefly then extinguishes
Cold air blowing instead of heat
Thermocouple looks dirty or corroded
Draft blowing out pilot light

Step-by-Step Diagnosis

Follow these steps to identify the exact cause of your problem:

Check if pilot light is completely out
Inspect pilot flame color (should be steady blue)
Verify gas valve is fully open
Check for drafts near furnace
Inspect thermocouple position relative to pilot flame
Look for dirt or debris in pilot assembly
Test if pilot stays lit when button held for 60 seconds
Check for soot buildup around pilot area
Verify adequate ventilation to furnace
Listen for gas flow when lighting pilot
Note if pilot goes out immediately or after minutes
Check for recent power outages or gas supply interruptions

What Causes It, Ranked by Likelihood

1. Failed or dirty thermocouple

The thermocouple holds the gas valve open while the pilot burns. When it wears out or gets sooty, the pilot lights but dies the moment you release the knob. That exact symptom is a thermocouple 9 times out of 10. The part costs $10 to $25, installed $100 to $250.

One of the most frequent culprits. You can usually fix this yourself, typical cost $20-50 (DIY) / $150-250 (professional).

2. Dirty pilot orifice blocking gas flow

A pilot flame that is small, yellow, or split instead of a steady blue cone points to a partly clogged orifice. The weak flame cannot heat the thermocouple enough to stay latched. Cleaning the orifice with compressed air during a service call restores it.

One of the most frequent culprits. You can usually fix this yourself, typical cost $0-20.

3. Draft or air current blowing out flame

A pilot that dies on windy days or when the blower kicks in is being blown out, often through a failed gasket, an uncapped chimney, or a drafty utility room. Watch the flame behavior with the burner door on versus off to confirm.

Worth checking after the common causes. You can usually fix this yourself, typical cost $0-100.

4. Bent or misaligned thermocouple

The thermocouple tip must sit directly in the pilot flame. A tip bumped during cleaning sits outside the flame envelope and never gets hot enough. Realigning it takes a pro minutes.

Worth checking after the common causes. You can usually fix this yourself at no cost.

5. Failed gas valve

If the thermocouple tests good and the pilot still will not stay lit, the electromagnet inside the gas valve is the remaining suspect. Valves run $300 to $800 installed, which on a furnace past 15 years usually starts the replacement conversation.

Worth checking after the common causes. This one needs a professional, typical cost $300-600.

6. Low gas pressure

Undersized or partially blocked gas supply, an overloaded meter, or utility-side pressure drops starve the pilot. If it happens when other gas appliances run, pressure is the lead. A tech measures inlet pressure with a manometer to confirm.

Less common, but it happens. This one needs a professional, typical cost $0-200.

7. Damaged or corroded pilot assembly

Rust and corrosion in the pilot hood distort the flame pattern until it cannot stay lit. Common in humid basements and coastal homes. Assemblies run $150 to $350 installed.

Less common, but it happens. This one needs a professional, typical cost $200-400.

8. Blocked flue or exhaust vent

A blocked flue changes draft through the burner compartment and can snuff the pilot, and it is also a carbon monoxide hazard. Check the vent termination outside for nests, leaves, or snow, and make sure your CO detector works before continuing to run the furnace.

Worth checking after the common causes. You can usually fix this yourself, typical cost $0-150.

DIY Solutions

You will need long lighter or matches, screwdriver, wrench set, compressed air, fine wire or needle, emery cloth (for cleaning). Plan on 30-60 minutes.

Turn gas valve to OFF and wait 5 minutes for gas to dissipate
Turn gas valve to PILOT position
Hold reset/pilot button down firmly
Light pilot with long lighter while holding button
Keep button pressed for 60 seconds after pilot lights
Slowly release button - pilot should stay lit
If pilot goes out, wait 5 minutes and repeat
Clean pilot orifice with compressed air or fine wire
Adjust thermocouple to be in blue part of pilot flame
Check thermocouple connection is tight
Remove dust and debris from pilot assembly area
Verify no drafts from nearby doors or windows
If pilot stays lit, turn gas valve to ON
Set thermostat to call for heat and verify furnace starts

Before you start, know these safety limits:

If you smell gas, leave immediately and call gas company from outside
Never light pilot if strong gas smell present
Ensure proper ventilation when working with gas
Don't force gas valves - can cause leaks
Carbon monoxide detector required when operating gas furnace
Never leave pilot lighting unattended
If unable to light after 3 attempts, call professional

When to Call a Professional

Stop and call a licensed technician if you run into any of these situations:

Strong gas odor that persists
Pilot won't light after multiple attempts
Yellow or flickering pilot flame
Thermocouple replacement needed and you're not comfortable
Gas valve appears damaged or stuck
Furnace is older than 20 years (consider replacement)
Soot buildup around burners or pilot
Carbon monoxide alarm triggered
Pilot goes out immediately after releasing button
Recent gas line work or pressure changes

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How to Keep It From Happening Again

Schedule annual furnace inspection and cleaning
Clean pilot assembly during yearly maintenance
Replace thermocouple every 3-5 years preventatively
Ensure proper ventilation in furnace area
Keep furnace area draft-free
Install carbon monoxide detectors on every floor
Address soot buildup immediately
Don't block furnace intake or exhaust vents
Test pilot light monthly during heating season
Keep furnace room door closed if drafty

Frequently Asked Questions

How do I know if my thermocouple is bad?

A faulty thermocouple causes the pilot to go out when you release the button, even if it lit successfully. The thermocouple generates voltage when heated by the pilot flame - if it's worn out, dirty, or misaligned, it won't keep the gas valve open. They cost $20-50 and last 3-7 years.

Is it dangerous if my pilot light keeps going out?

A pilot light going out occasionally isn't immediately dangerous - modern furnaces have safety features that shut off gas flow. However, repeated outages can indicate gas valve problems, draft issues, or incomplete combustion creating carbon monoxide. If it happens frequently, get professional service immediately.

Can I leave my furnace pilot light on all summer?

It's safe but wasteful to leave the pilot on year-round. Consider turning it off in summer to save $5-10/month in gas costs. Just remember to relight it before heating season. If you're not comfortable relighting it, the small cost may be worth the convenience of leaving it on.