How to Diagnose and Replace a Bad Flame Sensor

Posted by James Clark on Jan 9th 2023

How to Diagnose and Replace a Bad Flame Sensor

If you have noticed your furnace kicks on an off or fails to start up at all, one of your first diagnostic steps is to check the furnace flame sensor. This is one of the simplest furnace repairs; Oftentimes, a simple cleaning of the flame sensor will get you up and running again, however, there’s much more to flame sensors than that, and while a cleaning will get you through for a while, it is a part that does have to be replaced every so often through a furnaces life cycle.

Below, you will learn more about what a flame sensor is, where it’s located, how it works, and how to replace it when it goes bad.

What does a furnace flame sensor do?

A flame sensor is a device consisting of a metal element on a porcelain base that senses flames emitting from the burners. It is an absolute critical component of a furnace as ensures that the gas is ignited. Without this, if there was a problem with ignition, the furnace would dump gas in your home, causing a flame and explosion hazard.

What does a flame sensor look like?

Below you will see a photo showing the various styles of flame sensors you will encounter when servicing your furnace.

Examples of various flame sensors
Examples of Various Flame Sensors

Where is the furnace flame sensor?

The flame sensor will typically be mounted on the burner plate with the element in the path of the flames in between the burners and heat exchanger inlet and looks like a metal rod with one wire connected to it.

Typical Flame Sensor Location
Typical Flame Sensor Location

How do I know if I have a bad flame sensor?

When a flame sensor goes bad, the furnace may cycle on and off, or fail to start at all. On many furnaces, after 3 failed ignition attempts, the furnace will go into a lockout. One of the most common issues with furnace flame sensors is a build-up of dirt or other contaminants on the sensor. This build-up can interfere with the sensor's ability to accurately detect the flame, which can cause the furnace to shut off or malfunction.

Symptoms of a Bad Flame Sensor

  • Furnace Kicks on and Off
  • Furnace turns on and flames start, but then turns off
  • Flame Sensor is Sooty
  • Flame Sensor Element is Corroded
  • Flame sensor mounting surface is corroded
  • Porcelain base is cracked
Example of a Bad Flame Sensor
Example of a Bad Flame Sensor

Cleaning a Flame Sensor: Good in a Pinch, Better to Replace for the Long Term

While cleaning a flame sensor will get you through the night or longer, we advise replacing it once it has to be cleaned. The reason being, is that when a flame sensor is cleaned with abrasive materials, it exposes the raw metal to the air and acidic combustion gases, which accelerates corrosion on the flame sensor element. This combined with the increased surface area on the element from gouges from the abrasive material used to clean it, makes it far easier to corrode and fail again.

You may find yourself cleaning it far more often once it’s been cleaned once. Given how inexpensive a flame sensor is, we recommend replacing it once it’s failed once.

How to Replace or Clean a Bad Flame Sensor

Replacing a furnace flame sensor is one of the easier furnace repairs, but it is important to take safety precautions.

Before attempting to replace the furnace flame sensor, you will need to gather a few tools and materials. You will need a screwdriver, pliers, and/or a socket set as well as the new furnace flame sensor. You may also want to have a flashlight handy in case you need to see into tight spaces.

To replace the furnace flame sensor, follow these steps:

  1. Turn off the power to the furnace. We recommend turning the switch off next to the furnace, or if it’s plugged in, unplug it and then turn the power off at the breaker.
  2. Allow 30 minutes for the furnace to cool down.
  3. Remove the burner compartment door. There will be twist locks on the burner compartment door, or screws securing the panel. Other furnaces, you may simply have to lift up on the door, and pull the bottom outwards to remove the door.
  4. Locate the furnace flame sensor. It will look like a small metal rod and is mounted near the furnace burners. The flame sensor will be typically mounted at the last burner. It will be connected to the furnace's control board with a wire. Make sure you do not confuse this with the hot surface ignitor which looks more like a stick of gum or “stone” material.
  5. Dismount the old furnace flame sensor. Use the screwdriver to remove the screw(s) that hold the sensor in place.
  6. Right now is a good time to inspect the color of the flame sensor and burners. If you see excessive soot on the flame sensor and around the burners, your burners may be dirty and need replacing or the gas pressure is set too high, and causing the furnace to run too rich on fuel.
  7. Disconnect the wire from the old furnace flame sensor.
  8. If you are cleaning the flame sensor, use the following steps.
    1. When cleaning a flame sensor, you want to choose the least abrasive method possible to get it clean. Abrasive materials will expose bare metal, and create valleys and pits in the metal that allow particulates to collect on the flame sensor and cause it to fail sooner.
    2. What can I clean a In order of abrasiveness, these are the materials you can use to clean the flame sensor. Start with the least abrasive and work your way up until the corrosion and soot is removed.
      1. Cloth
      2. Soft-bristle brush
      3. Dollar bill
      4. Steel wool (Do not use a Brillo pad, as it will leave a tremendous amount of residue.)
      5. Scotch-Brite pad (This may leave more residue than other methods.)
      6. Wire brush
      7. Emory cloth (Use a fine grit. This leaves less particulates than sandpaper.)
      8. Fine-grit sandpaper (Last resort- make sure to remove all particulates left from sanding.)
    3. After removing the soot and corrosion, soak a lint-free cloth in alcohol and wipe the sensor clean.
  9. Connect the wire to the new furnace flame sensor.
  10. Mount the new furnace flame sensor. Use the screws that you removed in step 4 to secure the new sensor in place. Be sure to tighten the screws firmly to establish a good grounding surface. If the screw is not tightened tightly or the mounting surface is corroded, the flame sensor cannot complete its circuit through ground and will fail to work.
  11. Test the new furnace flame sensor. Turn the power back on to the furnace and wait for it to start up. After the burners start emitting flame, make sure that the flame sensor is in the path of the flame to be able to sense it effectively.
  12. If the furnace turns off after this step, check your wire connections, mounting surfaces, tighten the screw(s), and ensure that the flame sensor is in the path of the flame.

In conclusion, furnace flame sensors are an important safety feature of a gas furnace. They help to detect the presence of a flame in the furnace and to shut the furnace off if the flame is not detected so unburned gas does not enter the home. If you suspect that there is a problem with your furnace flame sensor, give us a call, and we will help diagnose and find the correct replacement flame sensor for your unit.

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About the Author

James Clark

James Clark is the HVAC Controls Specialist and Ecommerce Manager for Technical Hot & Cold. He has been with the company since 2014 and enjoys helping homeowners save money by providing help articles that walk them through various DIY HVAC repairs. In his spare time, he's playing music with his children and spending too much time working on his lawn.


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