Furnace Pilot Light Won't Stay Lit: Causes, Safe Checks, and When to Call a Pro

On an older gas furnace with a standing pilot, a pilot that won't stay lit is usually a worn or dirty thermocouple; Carrier calls it the most common cause, and Bryant calls it the most frequent. The thermocouple sits in the pilot flame and, as Bryant explains, sends a small electrical signal for as long as it senses heat; when it stops, the gas valve closes. Drafts, a clogged pilot orifice, low gas pressure, and a failing gas valve are the other usual suspects, according to Carrier. You can relight it by the label and stop drafts, but stop after two or three tries; the parts themselves are a technician's job.

First make sure you actually have a pilot. Most newer furnaces don't. If yours lights itself each time the heat comes on, see furnace not turning on and how to reset a furnace instead. For a gas water heater's pilot, see water heater pilot light won't stay lit.

First: Does Your Furnace Have a Pilot?

Carrier says pilot lights are found in older, traditional gas furnaces made before the mid-1990s, and newer furnaces use electronic ignition, such as a hot surface igniter or an intermittent pilot. Bryant's comparison puts standing pilots in furnaces made before 1995 and says a furnace made after the mid-1990s “almost certainly uses an electronic ignition system.” Other makers draw the line later: Lennox says most furnaces made since 2010 don't use pilot lights, and Trane says a furnace installed before 2010 may have one.

Two quick ways to tell:

• A standing-pilot furnace has a gas valve knob that Trane describes as marked Pilot, On, and Off, and a small flame that burns all the time, even when the heat is off.
• An electronic-ignition furnace says so on its label. Lennox's user manual for one of its gas furnaces reads: “This furnace is equipped with an ignition device which automatically lights the burners. Do not try to light the burners by hand.” If your label says that, there's no pilot to relight. Go to furnace not turning on.

Relight Only by the Label

The exact steps are on your furnace. Carrier says to follow the manufacturer's instructions printed on the furnace, and PG&E notes that basic relight instructions are often inside the main burner compartment door. Read them before you touch the knob. The common points across manufacturers and safety groups:

• Turn it off and wait. Bryant, Trane, and PG&E say to wait at least five minutes for gas to clear before relighting; the NFPA says five minutes or more. Carrier gives 3 to 5 minutes. If your label gives a longer time, use it.
• Hold the knob as long as the label says. Carrier explains that holding it heats the thermocouple, which signals the gas valve to stay open. Bryant says some standing-pilot furnaces need about 30 to 60 seconds.
• Stop after a few tries. Carrier says that if it won't stay lit after two or three attempts, stop and contact a licensed HVAC technician. Bryant says not to keep trying indefinitely.

Some utilities would rather you not relight it at all: DTE Energy says to contact a licensed professional if an appliance's pilot goes out. And the NFPA says that a pilot that repeatedly goes out or is difficult to light may mean there's a safety problem: “Do not try to fix the problem yourself. Have a fuel gas professional relight the pilot light.”

Most Likely Causes

1. A worn or dirty thermocouple. Carrier says a faulty thermocouple can incorrectly signal the gas valve to close, and Bryant says grime or wear can keep it from detecting the flame reliably. The classic sign is a pilot that lights while you hold the knob and goes out when you let go. Some furnaces use a thermopile, which Carrier describes as a more powerful version that generates enough electricity to run the gas valve on its own. Lennox says a technician is needed to fix a faulty thermocouple.
2. A dirty or clogged pilot orifice. Carrier and Bryant both list it: dust or debris partly blocks the tiny opening that feeds gas to the pilot, so the flame shrinks. Bryant adds that a flame that's too small, or that sits away from the thermocouple tip, may not send a strong enough signal. Carrier's troubleshooting table pairs a weak or yellow pilot flame with cleaning the orifice, work that belongs in the burner compartment with a technician.
3. Drafts. Carrier, Bryant, and Trane all name drafts. Bryant says vents, gaps in the furnace cabinet, or a nearby door can blow out the flame, and Carrier says a flickering or weak flame suggests a draft or low gas pressure.
4. The gas supply. Carrier lists low gas pressure from the supply line, and Bryant says too little gas can make the flame sputter and go out. If the gas was shut off at the meter, SoCalGas says not to turn it back on yourself; the utility turns the meter back on and relights the pilots.
5. A failing gas valve. Carrier lists a malfunctioning gas valve that fails to deliver consistent fuel, and Bryant says a pilot that won't stay lit can point to the gas valve or another safety part. That's a technician's diagnosis.

Thermocouple or Flame Sensor?

The two get mixed up online. Carrier says a thermocouple is used in older, standing-pilot furnaces and makes a small voltage when the pilot flame heats it, while a flame sensor is used in newer electronic-ignition furnaces to confirm the burner flame by passing a small current through it. The two parts are not interchangeable. A dirty flame sensor typically lets the furnace light and then shut off after 2 to 5 seconds, Carrier says, and Lennox says furnaces without pilot lights rely on that sensor to finish the heating cycle. If that sounds like your furnace, there's no pilot involved; see furnace short cycling. Carrier recommends having the flame sensor maintained by a trained HVAC professional.

What You Can Fix Yourself

• Check the flame. Carrier says a healthy pilot burns with a steady blue flame and a small yellow tip. A mostly yellow or orange flame means incomplete combustion, and Bryant warns that it may produce carbon monoxide. Trane says to call for repair if the flame is yellow.
• Stop drafts. Close a nearby door or window and make sure the furnace's access panels are in place, since Bryant names doors and cabinet gaps as draft sources.
• Clear the area. The NFPA says to keep anything that can burn at least 3 feet from the furnace and keep the area clean and uncluttered.
• Find your gas shutoff. PG&E says that if the pilot is out, shut the gas off at the appliance's gas shutoff valve; our shutoffs guide shows where it usually is.
• Relight by the label if you don't smell gas. If it won't stay lit after two or three tries, stop and call.

What Not to Do

• Don't keep relighting. Carrier draws the line at two or three attempts, and the NFPA says a pilot that keeps going out may be a safety problem.
• Don't take the pilot assembly apart. Replacing a thermocouple or cleaning an orifice is a technician's job; Lennox says a technician is needed for a faulty thermocouple, and Bryant recommends that all furnace repairs be done by a licensed HVAC professional.
• Don't turn a shut-off gas meter back on. SoCalGas says to leave it off and let the utility restore it.

When to Call a Pro

Call a licensed HVAC technician, or your gas utility for supply problems, when:

• The pilot won't stay lit after two or three tries, or keeps going out after relighting (Carrier, Bryant).
• The flame is yellow, orange, or weak (Carrier, Trane). Bryant recommends a professional inspection whenever the pilot flame looks abnormal.
• The furnace makes unusual noises after the pilot is relit (Carrier).
• You smell gas at any point. Leave first, then call from outside.

While you wait, an electric space heater can keep one room comfortable. The NFPA says to keep it at least 3 feet from anything that can burn, plug it straight into a wall outlet, and turn it off when you leave the room or go to bed; see our space heater guide. If you have to leave the house before the repair, the American Red Cross says to keep the heat no lower than 55°F so pipes don't freeze; see frozen pipes.

Prevent It

• Book a yearly inspection. The NFPA says furnaces need to be inspected and serviced at least once a year by a licensed professional, and the CPSC recommends having the heating system professionally inspected and serviced every year.
• Keep flammables away. Lennox's manual says not to store or use gasoline or other flammable vapors and liquids near the furnace.
• Put CO alarms on every level and outside sleeping areas, as the CPSC recommends; Bryant suggests one near the furnace, too. See our smoke and CO alarm guide, and if one sounds, what to do when a CO alarm goes off.

Frequently Asked Questions

Why does my furnace pilot go out as soon as I let go of the knob?

Most often the thermocouple: it isn't getting hot enough, or it's worn or dirty, so the gas valve closes. Carrier and Bryant both name it the most common cause. Hold the knob for the time on your label; if it still goes out after two or three tries, call a technician.

Can I replace the thermocouple myself?

The manufacturers here leave it to a technician. Lennox says you'd need a technician to fix a faulty thermocouple, and Bryant recommends that all furnace repairs be done by a licensed HVAC professional.

My furnace has no pilot. Why does it light and then shut off?

That's usually the flame sensor, not a pilot problem. Carrier says a dirty flame sensor can let the furnace start and then shut off after 2 to 5 seconds. See furnace short cycling and how to reset a furnace.

The Bottom Line

If you smell gas, leave. Otherwise, confirm the furnace really has a standing pilot, stop drafts, and relight it by the label. If it won't stay lit after two or three tries, or the flame is yellow, the thermocouple, orifice, and gas valve are a technician's job.

Sources

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