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Why Won't My Gas Furnace Stay Lit?

A single blue flame burning against a pitch black background

With cooler temperatures coming, many Arizonans are turning on their gas furnaces for the first time this year—only to discover that they won’t stay on! They ask themselves, “Why won't my gas furnace stay lit?”

In this blog, we’ll discuss the following so that you better understand how gas furnaces and pilot lights work and why yours won’t stay lit:

Gas furnaces and pilot lights are pretty complex, so they often require a lot of experience to diagnose and repair. Before we dive into the causes, we recommend reaching out to a technician as the easiest and safest way to get your heating system working.

Want a professional to diagnose your gas furnace issues? George Brazil is Phoenix’s most trusted HVAC company!

For nearly 70 years, our experts have helped Phoenix homeowners with their HVAC needs. That experience allows us to fix any furnace issue and leave you 100% satisfied, guaranteed.

Contact Arizona’s premier HVAC contractor today by calling us at (602) 842-0009 or booking below for no-cost diagnostic services.

How Gas Furnace Ignition Systems Work

A colored diagram showing the parts of a gas furnace system

Understanding the ignition system of gas furnaces is key when attempting to fix a pilot light that keeps going out. The typical system begins at the thermostat.

  1. At certain temperatures, the thermostat calls for heat.
  2. A draft inducer fan kicks on and pulls fresh air into the furnace to create a safe combustion chamber.
  3. Then, the gas valve opens and delivers gas to the pilot burner.
  4. An electronic spark ignites the gas, lighting the pilot light.
  5. The flame heats a flame sensor (like a thermocouple) that tells the furnace the pilot was successfully lit.
  6. Finally, the gas valve sends gas to the main burners, which the pilot light ignites.

Some furnaces are a bit different. For example, direct ignition furnaces don’t even have a pilot light, instead lighting the burners directly. Older furnaces have a standing pilot light, meaning you have to light them yourself.

Keep an eye on this process as it happens to help with identifying the source of your problems.

Common Issues With the Pilot Light

If your pilot light goes out without lighting the burner or if it never ignites, the issue could be:

Faulty Flame Sensor

Closeup of a hand holding the flame sensor rod pulled out of a furnace

Malfunctioning flame sensors cannot detect that the pilot light is lit. For safety reasons, the gas valve will close, and the fire will go out if the flame sensor cannot feel the pilot light. Flame sensors need regular maintenance, cleaning, and occasional replacement.

Incorrectly Positioned Flame Sensor

On many furnaces, the flame sensor is actively touching or is very close to the pilot light. If it’s out of position, it assumes the pilot light isn’t lit and shuts off the gas. In some cases, you might be able to reposition the flame sensor, though it may require replacing.

Malfunctioning Ignition Board

The ignition board is the component that controls the gas valve, electric spark, and flame sensor. When it fails, the whole system fails. Diagnosing issues with the ignition board can be extremely difficult without professional training.

Insufficient Gas Pressure

Closeup of a white info sticker on the side of furnace that shows model number, product number, etc.

Sometimes the issue isn’t with the heating system but the gas delivery. A pilot light requires a small but constant gas stream to stay lit. If gas pressure drops due to a leak or other issue, the pilot light can’t burn. You may be able to fix this by replacing the gas valve, but there could be issues elsewhere in the gas line.

Common Issues With the Burner

When the issue isn’t the pilot light, the burners may ignite and even run for a short period before going out. You might notice that the fan continues to run even after the pilot light goes out.


As a safety precaution, furnaces have limit switches that prevent the burners from burning too hot and causing major problems. Dirty filters, vent blockages, and other airflow issues can all cause the furnace to overheat, triggering the limit switch.

Faulty Limit Switch

Occasionally, the limit switch simply breaks and requires replacing. It pushes a false positive that the furnace is overheating, so the burners shut off.

Malfunctioning Thermostat

Someone using a unscrewing a piece from the exposed chip board on a thermostat affixed to a cream painted wall.

The problem with your furnace may lie in the thermostat. For example, regardless of what you set your thermostat to, it keeps telling the furnace that the house is warm enough and there is no need for more heat.

When To Seek a Professional’s Help

To prevent issues and ensure your furnace keeps working, contact a professional whenever your gas furnace doesn’t stay lit. Some causes of the problem are significant and require immediate professional care.

Hearing hissing or crackling sounds from your furnace can indicate a gas leak or electrical issue. Smells can also signify something serious. For example, rotten eggs mean leaking gas or a burning smell for electrical issues or fire.

Need Help Fixing Your Gas Furnace? Let George Brazil Help!

A man in a George Brazil uniform showing other uniformed techs an area on an opened furnace

If you have identified one of the above issues, you likely want a professional with fast, convenient scheduling. Book with George Brazil HVAC for a same-day gas furnace repair. Our certified professional technicians repair all makes and models.

Phoenix homeowners trust us to treat them with integrity because we value high-quality customer service. Over two thousand 5-star reviews, a 100% satisfaction guarantee, and a Better Business Bureau Torch Award for Ethics back up our work.

Start by calling us at (602) 842-0009 or booking below for a free diagnostic.