A Complete Guide to Furnace Flame Sensors (2024)

A flame sensor is one of the most important safety features of your furnace. This component is responsible for ensuring that your furnace doesn’t create a dangerous gas leak or explosion.

By understanding why the furnace flame sensor is crucial to the unit’s operation, you’ll be better prepared to recognize when a flame sensor is bad. Our guide will walk you through cleaning the sensor and furnace flame sensor replacement.

Suspect your flame sensor needs a repair? Connect with a local HVAC pro now.

What is a flame sensor?

The flame sensor is a safety component of a furnace located on the interior of the burner assembly. It monitors and verifies whether a flame is burning inside the unit.

If the sensor does not detect an active flame, then it will shut the furnace off to avoid a potential gas leak. The furnace flame sensor helps prevent explosions that could occur if gas were allowed to continue to flow into the home as well as carbon monoxide poisoning of those inside the home.

The Centers for Disease Control and Prevention report that at least 430 people die in the U.S. from accidental carbon monoxide poisoning each year and some 50,000 visit the emergency room because of the gas. While home furnaces are responsible for only a small percentage of these cases, it’s a worthwhile reminder to install a carbon monoxide detector on each level of your home.

Flame sensor vs. thermocouple

A flame sensor and thermocouple are often referred to as the same or interchangeable parts, but that’s not the case. While they both serve as a safety feature on the furnace, you would need to know which you have in order to replace or repair the component.

A thermocouple is often found on older model gas furnaces with a standing pilot. It has a small flame that burns continuously, keeping the top of the thermocouple at a high temperature. If the flame goes out and the tip of the thermocouple cools, it will automatically shut off the furnace’s gas valve.

Flame sensors are used in modern furnaces that operate via an electronic ignition rather than a standing pilot light. These models use electronic igniters to light the gas. As mentioned above, the flame sensors ensure the burners are operating properly and have ignited the gas. If the sensor detects that the burner has failed, it will turn off the gas.

Furnace flame sensor location

When trying to determine how to find a flame sensor, you’ll want to remove the furnace access cover to begin. Depending on the model of your furnace, you may have to remove screws or bolts to release the access cover.

The sensor is located just outside the burner assembly and looks like a small rod with porcelain surrounding the end that connects to the burner. Some furnace flame sensors are straight while others bend at a 45-degree or 90-degree angle. The sensor leads into the fire chamber where the flame burns.

Signs of a bad flame sensor

As with any other component in your heating and cooling system, the flame sensor can go bad. If your furnace stops working properly, look for warning signs that your flame sensor has failed:

  • Furnace burners turn on but go out after a few seconds
  • Soot and other debris is covering the end of the sensor (cleaning the flame sensor may fix this issue rather than a flame sensor replacement)
  • Tip of flame sensor is black
  • Porcelain/sensor casing is cracked

If a flame sensor replacement is necessary, it’s a low-cost fix that you can likely handle yourself if you feel comfortable working with the unit. 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? Book an appointment with a local expert now.

Flame sensor lifespan

If you’ve just replaced the sensor or had a new furnace installed, you can expect the flame sensor to last for about five years. The average lifespan of a furnace is 15 to 30 years, so you can anticipate a flame sensor replacement once or twice over the course of your homeownership.

To extend the lifespan of the sensor, seasonal furnace maintenance and cleaning the flame sensor is necessary. Even if the furnace seems to be operating smoothly, cleaning the sensor each year can prevent you from calling for emergency heating services in the middle of winter.

Furnace flame sensor cleaning

Before you begin the flame sensor cleaning process, turn the gas and electricity off to the unit. The valve handle on the gas pipe will allow you to stop the gas flow and turning off the furnace breaker at your electrical box will ensure all power is shut down.

If the furnace has been running, wait at least 30 minutes before attempting these steps:

  • Locate the flame sensor (refer to the furnace flame sensor location portion above)
  • Remove the furnace access cover, which may involve releasing screws or bolts
  • Unfasten/unscrew the flame sensor from the burner assembly
  • Disconnect the wiring that leads to the sensor and control box
  • Loosen the screws and remove the flame sensor
  • Use an emery cloth to gently remove soot and other debris from the sensor
  • Reattach the sensor
  • Reconnect the wiring to the sensor and control box
  • Secure the flame sensor to the burner assembly
  • Reattach the furnace cover access

Test the furnace to see if cleaning the flame sensor was the solution you needed. If the unit turns on and off properly, a flame sensor replacement may not be necessary.

Hire a pro to clean your flame sensor.

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Replace furnace flame sensor

During the sensor cleaning process, if you discover the tip of the sensor is black or the component is misshapen or broken in any way, replace rather than repair the part. Follow the flame sensor cleaning steps, but install the new sensor instead of cleaning the old one.

Your furnace will only operate with a certain type of flame sensor. Ensure the component you purchase is compatible with your unit. Check the owner’s manual for guidance on the type of sensor you need.

If working with a furnace or handling the gas line is not within your DIY comfort level, take the safe route and call an HVAC expert. A furnace specialist will test the flame sensor and replace it if needed.

To avoid being caught in a situation where furnace service is needed immediately, schedule routine inspections. Seasonal maintenance can support your unit in working properly when you need it most and expanding the lifespan of the furnace. Explore our top furnace brands if it’s time to replace your unit.

A Complete Guide to Furnace Flame Sensors (2024)

FAQs

Can I run my furnace without a flame sensor? ›

It's a thin, metal rod that monitors the pilot flame. If the pilot flame goes out, the flame sensor will turn the unit off to prevent natural gas from leaking into the home. Without the flame sensor, if the pilot light goes out but the gas valve remains open, the gas will not burn and will seep out of the furnace.

What is a good reading on a flame sensor? ›

Attach one wire from your multimeter to the flame sensor wire and the other one to the unit where you unplugged the wire. Turn on the unit. Look for a reading between . 5 and 10 micro-amps with the normal range between 2 and 6.

Are all gas furnace flame sensors the same? ›

There are different types of furnace flame sensors, but they all work in a similar way. They consist of small metal rods that are powered by electricity. During operation, individual gas valves will open. And each of the gas valves has a flame sensor.

What causes a flame sensor to go bad? ›

Possible Causes of a Flame Sensor Malfunction

The most common is that it is dirty. Because burning fuel of any kind produces trace amounts of moisture, over prolonged exposure to these conditions, metal will rust. Since your sensor is made of metal, rust can build up, preventing proper sensing of the flame.

Is it easy to clean a furnace flame sensor? ›

The sensor is easily accessible and typically mounted by one 1/4” hex head screw. Upon removing this screw, the sensor will slide out so that you can easily clean it. Carefully remove the sensor; it may be necessary to detach the wire fixed to the end of it to get more room to work, but this is not usually necessary.

How to reset furnace flame sensor? ›

Turning the power off for approximately 20 seconds and turning it back on again will reset most modern gas furnaces. Some furnace models can power on again after leaving the lockout for 1-2 hours. The cleanliness of the flame sensor of a furnace is very important.

Will a flame sensor cause a furnace not to ignite? ›

If your furnace won't ignite, the source of the issue may be a broken flame sensor. The sensor is responsible for turning off the gas burners to avoid gas buildup. Once this component starts to malfunction, it may stop the burners from igniting altogether.

Is a furnace flame sensor or ignitor? ›

Located in front of the burners, it's a rod that juts out where the flame comes out of the burner. It's always on the opposite side of the ignitor as well. For instance, the flame sensor is on the left side, while the ignitor is located on the right side of the burners.

What symptoms would a furnace with a dirty flame sensor exhibit? ›

Here are the main signs that the flame sensor in your furnace has gone bad and needs maintenance:
  • The heater doesn't turn on.
  • Short cycling. ...
  • Your furnace makes an intermittent clicking sound.
  • The flame sensor is noticeably dirty.
  • A crack forms on the porcelain exterior of the flame sensor.
Apr 14, 2022

What is the minimum acceptable reading for a flame sensor? ›

Now we want to check flame sensor voltage.

If the voltage is not within range there will not be enough electrical pressure to push the current through the flame. With your voltmeter, check the voltage between the sensor wire and the burner - it should be 90-107 volts AC.

How much should a flame sensor cost? ›

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?

How to troubleshoot a flame sensor? ›

Spotting a Faulty Furnace Flame Sensor
  1. Turn off the power.
  2. Turn off the gas valve.
  3. Remove the mounting screw.
  4. Carefully remove the sensor.
  5. Check the porcelain insulation on the furnace flame sensor.
  6. If it's burnt or dirty: clean it.
  7. If it's cracked or broken: get it replaced.
Oct 5, 2023

What are the three main types of flame sensors? ›

4 Common Types of Flame Detectors
  • Ultraviolet (UV) Flame Detectors. Ultraviolet (UV) flame detectors detect UV radiation the second a fire is ignited. ...
  • Infrared (IR) Flame Detectors. ...
  • Ultraviolet/Infrared (UV/IR) Flame Detectors. ...
  • Multi-Spectrum Infrared (MSIR) Flame Detectors. ...
  • Visual Flame Imaging Flame Detectors.
Aug 10, 2022

How do you reset the flame sensor on a furnace? ›

Turning the power off for approximately 20 seconds and turning it back on again will reset most modern gas furnaces. Some furnace models can power on again after leaving the lockout for 1-2 hours. The cleanliness of the flame sensor of a furnace is very important.

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