Quick summary
Match what you notice to a safe next step
These observations can narrow the starting service, but they do not confirm which part has failed.
What you notice
Dirty or failing thermocouple or thermopile
What it may point to
The safety sensor is not proving that the pilot flame is present, so the gas valve shuts off the pilot a few seconds to a minute after you release the button.
Safe next action
Note when the change began and whether the system is running
What you notice
Partially clogged pilot orifice or pilot tube
What it may point to
The pilot flame is too small, unstable, or pointed the wrong way, so it does not heat the sensor well enough to keep the pilot circuit open.
Safe next action
Compare nearby rooms and keep vents open
What you notice
Drafts or venting issues around the pilot assembly
What it may point to
Air movement is physically blowing the pilot off the thermocouple or extinguishing it, which often shows up as a pilot that lights normally but drops out intermittently.
Safe next action
Record any code, sound, smell, or weather event
What you notice
Weak safety magnet or internal failure in the gas valve
What it may point to
Even with a good pilot flame and sensor signal, the valve cannot hold the pilot open, so the flame dies as soon as the control knob is released.
Safe next action
Check the filter and arrange an evaluation if it persists
Safe things to check
- Confirm the thermostat mode and setpoint
- Inspect the air filter and replace it if visibly dirty
- Make sure visible supply and return vents are open
Do not attempt
- Do not adjust gas valves, burners, ignition parts, or venting
- Do not open sealed electrical or equipment panels
- Do not bypass a safety switch or force repeated restarts
A few quick questions
Refine the best next step
Question 1 of 3
- 1How much of the home is affected?
- 2What is the system doing right now?
- 3When did this begin?
What this symptom can mean
Use the evidence you can observe
What this symptom usually means
A pilot light that will light but not remain lit is a very specific failure: the furnace is losing its proof-of-flame signal at the pilot assembly. On older standing-pilot furnaces, the pilot flame must stay strong enough to heat a thermocouple or thermopile continuously. If that sensor cools down, or if the gas valve cannot hold on the signal it receives, the pilot gas shuts off and the furnace cannot proceed to normal burner operation.
What you can notice before a technician arrives
If the flame goes out immediately when you release the pilot button after holding it for the normal lighting time, the thermocouple, thermopile, or gas valve is high on the list. If the pilot flame looks tiny, wavering, yellow instead of mostly blue, or does not wrap around the sensor tip, a dirty pilot orifice is more likely. If the pilot stays on for a while and then drops out when the blower starts, when a basement door closes, or during windy weather, air movement or venting problems become more likely than a simple ignition issue.
Why this is different from other furnace no-heat problems
This symptom is not the same as a furnace that clicks but never ignites, blows cold air, or has a blower problem. In this case, the failure is centered on the small pilot flame and the safety circuit that must keep that flame proven. Many modern furnaces do not use a standing pilot at all, so if your equipment has electronic ignition, a homeowner may describe the problem this way even though the actual repair ends up involving the igniter or flame-sensing sequence rather than a true pilot assembly.
What a service visit typically includes
A qualified technician will verify whether the furnace has a standing pilot or an intermittent pilot system, inspect the pilot flame shape and color, check how the sensor sits in the flame, test thermocouple or thermopile output, and confirm whether the gas valve is holding properly. They may clean the pilot orifice, correct flame impingement, check for draft interference, inspect venting near the furnace, and measure incoming and manifold gas pressure. Gas valve and burner work are not homeowner tasks.
What affects repair cost
Cost depends heavily on whether the fix is a cleaning/adjustment or a parts replacement. In many US markets in 2025-2026, cleaning a pilot assembly and confirming operation often falls around $170-$575 total, while replacing a thermocouple or thermopile is commonly about $170-$575 depending on access and part type. If testing shows a bad gas valve, total repair can land closer to $500-$1,575 or more, and age of the furnace may affect whether replacement is worth discussing.
When to turn the system off
- Stop if you smell gas, see smoke or sparks, or hear a carbon-monoxide alarm
- Stop if water reaches powered equipment or a breaker trips again
- Stop if loud grinding, booming, or heavy ice appears
When to call for help
- Call when the symptom continues after the safe checks
- Call promptly when comfort is lost for the whole home
- Describe when it started and any water, ice, noise, odor, alert, or breaker activity
Cost guidance
What could this cost to fix?
Price depends on the actual cause, equipment, access, and local labor. Compare the national ranges in the Furnace Repair Cost guide, or check service availability for your system.
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Browse HVAC help by stateFrequently asked questions
Furnace pilot light will not stay lit: common questions
Why does my pilot light go out when I let go of the button?
Can a draft really make a furnace pilot light go out?
Is a pilot light that will not stay lit usually an expensive repair?
Can I keep trying to relight the pilot myself?
How long does it take a technician to diagnose a pilot that will not stay lit?
Sources and review standard
Last updated July 18, 2026. Last reviewed July 18, 2026. HVAC Rescue reviews problem pages for homeowner-safe boundaries, service routing, source quality, and unsupported claims. No named technical reviewer is shown unless a qualified HVAC reviewer has completed that review.
