Why does my gas furnace flame keep going out?

Author: Braxton Ullrich  |  Last update: Saturday, April 19, 2025

Airflow through the furnace comes from the return air ducts and must pass through a filter (which keeps dust and debris from getting into the furnace). This filter will become clogged after one to three months, and unless it's replaced with a clean one, it can cause the burners to go out early.

Why does the flame on my furnace keep going out?

For the burners to ignite, they need airflow. The airflow into the furnace comes from the blower fan that draws air through the return air ducts. But if this airflow is choked off because of a clogged furnace filter or faulty blower, the burners may not be able to stay on for long.

Why does my furnace ignite for a few seconds then shut off?

The flame sensor does exactly what it sounds like, it senses when the flame of the furnace is on and sends a message to the control board to start blowing warm air. If dirt or dust covers the sensor, the flame isn't detected, and the furnace shuts down.

Why does my gas flame keep going out?

There are many possible reasons why the burner will not stay lit - the gas jet may be blocked, the valve may be set too low, the air flow may be restricted, there may be an issue with the pilot light, and so on.

How do you tell if you have a bad flame sensor?

Top 7 Signs of a Bad Furnace Flame Sensor
  1. Flue Line Corrosion. The flue line is responsible for venting harmful gases from the furnace outdoors so nothing seeps into the house. ...
  2. High Energy Bills. ...
  3. Yellow Flames. ...
  4. Furnace Short Cycling. ...
  5. Dirt on the Sensor. ...
  6. Visible Damage on the Flame Sensor. ...
  7. Age.

TOP 10 Reasons Why the Gas Pilot Light Goes Out & Won't Stay Lit!

Why does my furnace flame sensor keep going bad?

The flame sensor should not have visible dirt or corrosion on it. If you don't get routine maintenance for the furnace, then it's more likely you will develop these problems. Your technician will otherwise clean off the sensor every year when they perform the maintenance.

How much does it cost to replace a flame sensor?

Most furnace flame sensors cost less than $20, but if you opt to have a professional HVAC technician make the repair, you can expect to pay between $75 and $250. Need help replacing your flame sensor?

Why is my furnace igniting but not staying lit?

Dirty flame sensor

If this sensor is dirty, the dirt can cause the sensor to operate intermittently. When this happens, your furnace will most likely light, but because the sensor can't properly detect if a flame is present. The sensor may signal the ignition cycle to end, which your furnace won't stay lit.

Why is my gas flame not staying on?

The most common cause of this is dirty or blocked burners, which is the part that you can easily take off to clean.

Why does my gas fire keep going out?

Your gas fireplace could be turning off for several reasons. The most like culprits are likely tied to some kind of problem with your setup. Anything from issues with your pilot light, clogged ports on burners, damage to heating components, and thermocouple/thermopile malfunctions can be to blame.

Why does my furnace only stay lit for a few minutes?

To begin, ensure that all the filters of your furnace are clean. Also, get the filters replaced every few months to ease the burden on your HVAC system and ensure that fresh air comes filtered into your home. Another common cause of your furnace turning on and off after a few minutes is a faulty pressure switch.

Why does my furnace only ignite sometimes?

The Furnace will only ignite if the thermostat is functional. Double-check that the temperature is set higher than the temperature of the surroundings. If you have a programmable device, check for an error code. Once you've inquired about all these matters, restart your thermostat.

Why does my furnace turn on for a few seconds then shut off?

There are multiple reasons why your furnace is short cycling. These could include a dirty filter, blocked vent, or malfunctioning flame sensor. You can switch for a clean furnace filter, or have your HVAC technician resolve a more complicated issue.

What does a damaged thermocouple look like?

Failed Thermocouple Symptoms

The first is signs of contamination on the tube, which can include discoloration, cracks, or pinholes. Next, check the wiring for any signs of wear or corrosion like missing insulation or bare wire.

Why does the flame keep turning off on my furnace?

A dirty or corroded flame sensor can cause your furnace to keep shutting off unexpectedly. The flame sensor detects if the furnace's burner is lit, and if it doesn't register the flame, it will shut off the gas valve to prevent a gas leak.

Why is my gas fire not holding a flame?

So you've checked your pilot light and even reset it, but still, it wont stay lit. This can be an indication that your fireplace thermocouple is either bad or worn out. However, if your pilot is lit but the gas fireplace isn't, then there's probably a problem with your thermopile.

Why wont my gas heater flame stay lit?

If the pilot light of your gas heater fails to ignite or stay lit, it may be due to a dirty pilot assembly. Over time, the pilot assembly can accumulate dust, debris, or other contaminants, which can obstruct the flow of gas or impede the proper functioning of the flame.

How to clean a thermocouple on a gas heater?

How to Clean a Thermocouple
  1. Locate the pilot assembly and find the thermocouple. ...
  2. Use an adjustable wrench as needed to remove thermocouple from the gas valve.
  3. Detach the thermocouple from the pilot assembly. ...
  4. Gently scrub the thermocouple with the abrasive side of a sponge or scouring pad to remove the soot.

How do I get my furnace to stay lit?

Let's start first by assessing if you can quickly resolve your pilot light issue with basic troubleshooting.
  1. Adjust your thermostat. Ensure that your thermostat is set to HEAT and ON because your furnace will simply not turn on if your thermostat is set to COOL. ...
  2. Replace your air filter. ...
  3. Check your circuit breaker.

How to reset furnace flame sensor?

There is a work-around to waiting. If there is no safety risk (like a gas leak), a furnace can be reset to bypass this wait. Turn off the furnace's power switch and wait 10 seconds before turning it back on.

How do I know if my furnace flame sensor is bad?

6 Signs of a Malfunctioning Gas Furnace Flame Sensor
  1. Yellow Burner Flames. Open up or glance through the peephole of your gas furnace to see the color of the flames firing up in the burner. ...
  2. Rusted Flue Lines. ...
  3. High Utility Bills. ...
  4. Old Age. ...
  5. Exhaust Smells. ...
  6. Constant Thermostat Adjustments.

What is the most expensive part to fix on a furnace?

A heat exchanger replacement is often one of the most costly repairs in a furnace, and the expense can sometimes rival that of installing a new furnace. Typical replacement costs vary widely but can easily reach several thousand dollars, especially if labor costs are factored in.

What is the life expectancy of a furnace flame sensor?

While most furnaces last for 10 to 20 years, flame sensors have a much shorter lifespan of about five years. Usually, when a flame sensor goes bad, the best route is to replace it outright.

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