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.

This is gas work, not a DIY job. A licensed technician should service the furnace, even on a standing-pilot unit.

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.

Skip the parts shopping on this one. A thermocouple sits in the gas path, so a licensed technician should source and install it. See the 2026 cost range above for what a service call runs.

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.

What the pro will do

There is no do-it-yourself path we recommend for this repair. The safe and lasting fix comes from a licensed pro who can diagnose the true cause and complete the work correctly.

Booking a qualified tech protects your safety, your warranty, and your home. Call a licensed professional to handle it end to end.

What it costs in 2026

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.

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.

This isn't a weekend project. Anything touching the gas valve or pilot assembly belongs with a licensed technician.

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.

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.

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.