Why won't my gas furnace pilot stay lit in summer?

The direct answer

Your pilot lights, burns while you hold the knob, then dies the moment you let go. That's the classic thermocouple story, and it's true maybe seven times out of ten.

The thermocouple is a pencil-thin metal probe that sits in the pilot flame. When it heats up, it makes a tiny electrical signal that tells the gas valve, "the flame is real, keep the gas flowing." When it weakens, the valve stops trusting it and shuts the gas off. Pilot dies.

The second most common cause is a dirty pilot orifice. After a furnace sits idle all summer, dust and cobwebs choke the tiny opening, the flame goes weak and yellow, and it can't heat the thermocouple enough.

Good news: both are cheap and mostly DIY on a standing-pilot furnace. A pro visit runs $150–$700, mostly because they may find the gas valve is the real problem.

What's actually going wrong

Think of this as a short chain: gas gets to the pilot, the pilot flame heats the thermocouple, the thermocouple holds the valve open. A break anywhere in that chain drops the pilot.

Start at the thermocouple because it's the weakest link by design and the cheapest to replace. These probes just wear out. Ten to fifteen years is a normal lifespan, and a lot of standing-pilot furnaces are well past that.

If a new thermocouple doesn't fix it, back up to the gas flow. A partially clogged orifice starves the flame. You'll see a lazy yellow flame instead of a crisp blue one, and it won't wrap the thermocouple tip the way it should.

The connection point matters too. The thermocouple screws into the gas valve with a small nut, and if it's loose (common after any past service work), the signal gets weak even when the part is fine.

Drafts are the sneaky one in summer. An open basement window, a running dryer, or a bathroom exhaust fan can pull enough air through the space to snuff a small flame. If your pilot flickers visibly, look for airflow before you condemn any parts.

The gas valve itself is last on the list because it's the most expensive and the least likely. When the magnetic coil inside weakens, no thermocouple on earth will hold it open. That's a pro diagnosis and often a replace-the-furnace conversation.

Working through it step by step

Give yourself a clear bench. Pull the burner access panel and find the lighting sticker. Every furnace has slightly different knob positions, so read yours rather than trusting memory.

Turn the gas control to OFF first and wait a full 5 minutes. This isn't optional. It clears any gas that leaked into the box so your relight doesn't turn into a small fireball.

Now relight per the instructions and hold that knob down for 60 seconds. Count it out. The number one reason a DIY relight "fails" is letting go at 20 seconds.

Release and watch. If it holds now, great, you probably just needed a full heat-up cycle or you knocked debris loose. If it dies, you've confirmed the thermocouple or valve is the issue.

Look at the flame quality on your next attempt. Strong blue cone means gas flow is fine and you're chasing the thermocouple. Weak yellow means clean the orifice first.

To clean: gas off, let it cool, and use canned air or a soft brush on the pilot opening. Reseat the pilot tube if it looks off-angle. Then relight and reassess.

Replacing the thermocouple is the payoff step for most people. Note how it routes, unthread it at the valve, slide it out of the pilot bracket, and reverse with the new one. Match the length when you buy it.

If you've done a new thermocouple, cleaned the orifice, and snugged the connection, and it still won't hold, you're done as a DIYer. That's the gas valve talking, and it's time for a licensed tech.

• Access panel off, lighting sticker located
• Gas OFF, 5-minute wait
• Relight, hold knob 60 seconds
• Watch flame color and whether it holds on release
• Clean orifice if flame is yellow or weak
• Snug the thermocouple nut at the valve
• Swap thermocouple if flame is blue but won't hold
• Stop and call a pro if all of the above fail

What it costs in 2026

That covers a universal thermocouple ($8-$20), maybe a can of compressed air, and a basic wrench if you don't own one. Most of that budget is buying tools you'll keep.

A pro visit runs $150–$700 in most US markets. The service call and diagnostic is usually $90-$180 of that, and the rest depends on what they find.

A pro thermocouple replacement lands around $150-$250 all-in, which is the low end of that pro range. You're mostly paying for the trip and the peace of mind of a gas-safe hands.

If it turns out to be the gas valve, expect $350-$650 for the part and labor. On a standing-pilot furnace, that's the moment to get a replacement quote before spending it.

Venting or backdraft problems can run higher because they need a combustion analyzer and sometimes flue work. That's not a fixed-price item, so get the diagnosis first.

DIY or call someone

This is one of the friendlier gas repairs for a careful homeowner, with clear limits.

Do it yourself if: your furnace has a standing pilot, you can smell nothing unusual, and the fix is a thermocouple swap, an orifice cleaning, or tightening a connection. Those are low-risk with the gas off.

Call a pro if: you replaced the thermocouple and it still won't hold, you suspect the gas valve, you notice a persistent draft or backdraft, or you smell gas at any point.

Also call a pro if your furnace has electronic ignition rather than a standing pilot. If there's no continuously burning pilot flame to speak of, this whole diagnosis changes and the parts (igniters, flame sensors, control boards) are different animals.

There's no shame in the phone call. Working near a live gas valve is a reasonable place to want a licensed set of eyes.

Can this wait?

In summer, yes, mostly. You don't need heat, so a dead furnace pilot isn't an emergency the way a July AC failure is.

The catch is that summer is exactly when you should fix it, precisely because you have time. A thermocouple that fails now will fail worse on the first cold night, when every HVAC company in town is slammed and charging premium rates.

Two things change the math. If you smell gas, it's not a wait situation, handle it immediately per the safety notes. And if the pilot flame is weak and yellow, get the venting checked before running the furnace under load, because that can hint at a combustion problem.

For a straightforward thermocouple or orifice issue with no gas smell, treat it as a relaxed weekend project. Just don't let it slide until October.

If your furnace still has a standing pilot

A furnace with a continuously burning pilot is old. Most were installed before the mid-1990s, when electronic ignition took over for efficiency reasons.

That age matters for two reasons. First, the parts are simple and cheap, so small fixes stay cheap. Second, the whole unit is likely running at 60-70% efficiency, versus 90%-plus on a modern furnace.

So when a repair crosses a few hundred dollars, pause. Sinking $500 into a gas valve on a 28-year-old furnace rarely pays off. That money is a down payment on a unit that'll cut your winter gas bill.

None of that means you scrap it over a $12 thermocouple. Small fix, do it. Big fix, get a replacement quote first and compare.

Why summer is the usual suspect window

Standing pilots run all year, even in July. That constant tiny flame is convenient in winter but pointless in summer, and it's exposed to whatever settles in the burner box during the long off-season.

Spiders are a real thing here. They're drawn to the trace odorant in natural gas and spin webs right across pilot orifices and burner ports. each spring, homeowners often clear a few.

Basement air is different in summer too. Open windows, dehumidifiers, and cross-ventilation create small drafts that a winter-sealed house doesn't have. That's enough to nudge a marginal pilot over the edge.

Humidity accelerates corrosion on the thermocouple tip and the pilot bracket, which slowly drops the signal until one hot afternoon it just won't hold.

How we put this together

This guide reflects standard field diagnosis for standing-pilot gas furnaces, ordered by how often each cause actually turns up on a service call.

Cost ranges are 2026 US estimates blending national parts pricing with typical regional labor rates. Your local market can swing the pro numbers 20-30% either direction.

We draw a hard line at the gas valve for DIY work and at any gas smell, because those cross from inconvenience into safety. When in doubt, the right move is a licensed HVAC technician, not a YouTube workaround.