Furnace pilot light keeps going out? Here is why and what to do

Why does my furnace pilot light keep going out?

The most common answer by a wide margin is a worn or dirty thermocouple. The thermocouple is a thin metal probe whose tip sits right in the pilot flame. When the flame heats it, it generates a tiny electric current, and that current is what tells the gas valve the pilot is actually lit so it is safe to keep the gas flowing. When the thermocouple is dirty, bent slightly out of the flame, or simply worn out, it stops generating enough current, the valve treats that as a pilot that has gone out, and it shuts the gas off as a safety. That is exactly why the pilot burns fine while you hold the knob down but dies the instant you release it.

A few other things put a pilot out, and the flame itself tells you which. A strong, steady blue flame that dies on release points at the thermocouple. A weak, small, yellow, or flickering flame points somewhere else, usually a dirty pilot orifice or a draft, covered below. A pilot that will not light at all, even while you hold the knob, points at the gas supply rather than the thermocouple, because the thermocouple only matters once there is a flame to heat it. Reading the flame before you start swapping parts saves you time and money.

How do I relight my furnace pilot light safely?

Before anything else: if you smell gas, a rotten-egg or sulfur smell, do not relight, do not flip any switch, leave the house, and call your gas company from outside. That overrides everything below. If there is no gas smell, you can safely relight a standing pilot once or twice following these steps.

• Turn the thermostat down and the gas knob to off. Find the gas control valve on the furnace, a knob marked On, Off, and Pilot, and turn it to Off.
• Wait at least five minutes. This is the step people skip and the one that matters most. Waiting lets any unburned gas clear out of the chamber before you introduce a flame, so you are not lighting a pocket of gas.
• Turn the knob to Pilot and hold it down. Holding the knob manually feeds gas to the pilot. Light the pilot with the igniter button or a long lighter while you keep holding.
• Keep holding for a full sixty seconds. This heats the thermocouple enough to take over. Letting go too soon is the single most common reason a relight fails, so give it the full minute.
• Release slowly and watch. If the pilot stays lit, turn the knob to On and you are done. If it dies, the thermocouple is the likely cause. Try once more holding the full sixty seconds, and if it still will not hold, stop and call a technician.

The reason to stop after a try or two is not just frustration. Each failed attempt puts more gas through the chamber, and repeatedly relighting a pilot that will not hold masks a real fault that needs a professional, whether that is the thermocouple, the gas valve, or a venting problem.

What if the pilot flame is weak or yellow instead?

A healthy pilot is a strong, steady blue flame that wraps around the tip of the thermocouple. If yours is small, weak, or flickering, the usual cause is a dirty pilot orifice, the tiny opening that feeds gas to the flame. Over years it clogs with dust and carbon, the flame starves, and a starved flame is both too weak to stay lit and too cool to heat the thermocouple. The same weak flame can also be caused by a draft, which is why a pilot that only goes out in windy weather, when the furnace panel is off, or when a big exhaust fan runs, is often a draft problem rather than a parts problem.

A flame that burns yellow or orange instead of blue is different, and it is a warning, not just a weak flame. Yellow means the gas is not burning completely, and incomplete combustion can produce carbon monoxide. A pilot or burner flame that is consistently yellow is a reason to shut the furnace off and call a technician rather than keep relighting it. Do not treat a yellow flame as a simple relight.

Could it be a gas supply or a draft problem instead?

If the pilot will not light at all, or lights but burns weakly across the board, the issue may be the gas getting to it rather than the parts at the pilot. A gas shutoff valve that is only partly open, a drop in line pressure, or a regulator problem all starve the flame. There is a quick way to tell this apart from a furnace fault: check another gas appliance, like your stove or water heater. If those are also acting up, the problem is your gas supply, not your furnace, and the call is to the gas company. If you heat with propane, check the tank gauge first, because a near-empty tank drops the pressure to everything in the house.

Drafts are the other outside cause, and they are easy to confirm by pattern. A pilot that relights fine and only blows out during windy weather, when the furnace access panel is off, or when a powerful exhaust fan runs, is being snuffed by moving air rather than failing on its own. A more serious version is a downdraft in the flue, where air comes back down the vent instead of letting exhaust rise. That not only blows out the pilot but can pull combustion gases back into the house, which is a venting problem and a carbon monoxide concern, not a simple relight. A pilot that keeps dying with no wind and no fan running is worth having a technician check the venting for exactly this reason.

My furnace has no pilot light. What am I actually dealing with?

This catches a lot of people, so it is worth checking before you go looking for a pilot. Furnaces built since roughly 2010 usually have no standing pilot at all. Instead of a small flame burning around the clock, they light the gas only when heat is called for, using an electronic igniter that either glows orange or sparks, and they confirm the flame with a part called a flame sensor rather than a thermocouple. If you watch your furnace start and see a part glow orange or hear rapid clicking just before the burners light, you have electronic ignition and there is no pilot to relight.

On those furnaces, the symptom that feels like a pilot going out, the burner lighting and then shutting off a few seconds later, over and over, is almost always a dirty flame sensor, not a thermocouple. The fix is different and so is the page: our guide on a furnace that short cycles covers the flame sensor and the other reasons a modern furnace lights and quits. The rest of this page is about older furnaces that have a true standing pilot flame.

When is a pilot problem a carbon monoxide danger?

This is the part to take seriously, because a furnace that is not burning cleanly can produce carbon monoxide, a gas you cannot see or smell. A few signs raise a pilot problem from a simple fix to a safety issue: a flame that burns yellow instead of blue, soot or black marks around the burner, or a pilot that keeps going out because the flue is backdrafting and pulling combustion gases back into the house. Any of those means stop and call a professional. The same goes for the worst case behind repeated combustion problems, a cracked heat exchanger, which leaks carbon monoxide into your warm air and cannot be safely repaired, only replaced.

Two rules protect you here. First, every home with a gas furnace needs working carbon monoxide alarms, one on each level and near the bedrooms, because the alarm is your only warning for a gas you cannot sense. If a CO alarm sounds, get everyone and any pets outside to fresh air, then call 911 from outside, and do not go back in until responders say it is safe. Carbon monoxide poisoning looks like the flu, headache, dizziness, nausea, but without a fever, so do not wait it out. Second, a yearly furnace inspection is the cheapest way to catch a combustion or venting problem before it becomes dangerous.

Should I turn the pilot off in summer?

Yes, on a furnace with a standing pilot it is both safe and sensible to turn the pilot off for the summer, by turning the gas control knob to Off. A standing pilot burns gas around the clock all season whether the furnace runs or not, so shutting it off while you are not heating saves a little gas and removes any small chance of a pilot-related issue while the system sits idle. The only thing to plan for is relighting it in the fall, which is the same procedure above. Check the label on your furnace, since a few manufacturers recommend leaving the pilot lit, but for most older units, off for the summer is fine.

What do these repairs cost, and is my furnace too old to fix?

A pilot that will not stay lit is usually a cheap repair, which is the good news. These are installed prices, parts and labor together.

Repair	Installed cost	What it fixes
Thermocouple replacement	$100 to $250	The probe behind most pilots that will not stay lit. The most common fix.
Pilot assembly or orifice cleaning	$100 to $300	Clears the clog behind a weak or yellow flame, often part of a tune-up.
Flame sensor (electronic-ignition furnaces)	$100 to $350	The thermocouple's modern equivalent on a furnace with no pilot.
Hot surface igniter	$150 to $350	The part that lights the gas on a newer furnace when it fails.
Gas valve replacement	$200 to $800	The valve itself, which can drop a pilot even with a good thermocouple.

Here is the bigger question hiding behind a recurring pilot problem. A furnace that still has a standing pilot is usually twenty to thirty years old, and a furnace that old is often only about 65 to 70 percent efficient, meaning roughly a third of the gas you pay for goes up the flue. A new furnace runs no standing pilot, wastes far less, and starts at 80 percent and climbs past 90. So if you are sinking repeated repair calls into an old furnace that keeps losing its pilot, it is worth pricing a replacement rather than the next part. Our replace-versus-repair calculator helps you weigh the repair bill against a new unit, and our guide on the signs you need a new furnace covers when age tips the decision.