How To Clean Flame Sensor on AO Smith Water Heater

The flame sensor, also known as a thermocouple, is a device on gas water heaters used to control gas flow. Over time, a flame sensor can get dirty and corroded. Cleaning the flame sensor on an A.O. Smith water heater will ensure that it doesn’t rust due to carbon buildup. 

Here’s how to clean the flame sensor on an A.O. Smith water heater:

  1. Gather the tools you need.
  2. Disconnect the water heater and shut off the gas.
  3. Remove the burner assembly.
  4. Inspect the flame sensor.
  5. Gently rub the flame sensor with an emery cloth. 
  6. Reinstall the burner assembly.
  7. Confirm that the water heater is functioning correctly. 

The flame sensor is critical because without it, the burner will not sense the presence of a flame, and it will not work. Read further to find out how to clean the flame sensor. 

AO smith water heater Flame Sensor

1. Gather the Tools You Need

Before opening up the burner and separating the parts to access the flame sensor, you must collect all the necessary tools. 

Here are the tools you need for cleaning the flame sensor on an A.O. Smith water heater: 

  • Emery cloth or fine steel wool
  • Screwdriver
  • Flashlight

Some water heaters have Torx screws, so ensure you have a Torx screwdriver handy in case you need it.

The Amazon Basics Screwdriver Bit Set has a variety of Philips, Slotted, and Torx screwdrivers. They are portable, come in various sizes, and packaged in a plastic case, so you need not worry about losing them. 

Once you have all the necessary tools, you can remove the parts to get to the flame sensor.

2. Disconnect the Water Heater and Shut off the Gas

Before getting to work on the water heater, switch it off, unplug it, and shut off the gas supply. Shutting off the gas supply typically involves turning the lever in a clockwise position but it can be different, depending on the model you have.

You don’t want to risk working on the heater when the gas valve is on. When you disconnect the gas supply, unscrew and remove the cover plate.

disconnect water to AO smith water heater

3. Remove the Burner Assembly

The burner assembly varies depending on the model. For example, some burner assemblies have a fusible link while others don’t. The other parts in the burner assembly include the switch, pilot light, and flame sensor (flame rod). 

The flame sensor senses the flame from the pilot light. As the flame sensor heats up, it sends a signal to the gas valve, letting it know there is a flame, so the water heater should start running. 

The main gas valve then opens up and releases enough gas to run the water heater. Unfortunately, as the flame blasts on the flame sensor, it leaves soot, which builds up over time. 

A dirty flame sensor will eventually stop working correctly because it will not “sense” the pilot flame as it should. Sometimes, the flame sensor assumes it is receiving a low wavelength flame and sends the wrong signal to the gas valves.

Unscrew the flame sensor and pull out the rod. You don’t need to remove other parts of the burner assembly. 

If you need additional guidance, this video is a great guide on how to remove the burner assembly in an A.O. Smith water heater.

4. Inspect the Flame Sensor

Before you start cleaning flame sensor on your A.O. Smith water heater, you should inspect it, and check how much soot has collected. 

If you are cleaning the flame sensor for the first time, you may not be able to see how bad the carbon buildup is and if you should be concerned about other things, such as the combustion rate. 

When the water heater is working correctly, you only need to clean the flame sensor once a year during regular maintenance. However, if you clean the flame sensor too often, there could be other underlying issues:

  • A leaking gas valve.
  • A fault in the combustion process, such as burning the wrong amount of fuel. 
  • The water heater has no cover, causing the impurities in the air to collect on the flame rod. 
  • Indentations on the flame rod. 

You should dig deeper if the flame rod collects a lot of dirt within a short time. A sharp tool or harsh abrasive may have gouged out the flame sensor. The indentations collect dirt faster, so, you should also look out for signs of damage to the flame sensor.

Related Posts:

5. Gently Rub the Flame Sensor With an Emery Cloth

Wrap an Emery cloth around the flame sensor and rub it until you remove all the carbon build-up. You shouldn’t use sandpaper because it contains silica. 

After doing this, you should have a clean and shiny flame sensor. However, it will also have silica residue, which turns to glass when exposed to heat. 

Now, instead of only worrying about the carbon buildup, you will have a flame sensor with a glass coating. 

Emery cloth has multiple grits, so you can choose one that is most suitable, depending on the amount of carbon residue on the flame sensor. Like sandpaper, the lower the grit number, the more abrasive it is. 

The LYJFXX Emery Cloth Roll (available on has assorted grits, from coarse to fine, that you can use to clean the flame sensor. It is wear-resistant and anti-stretch, and you can use it on different types of metals and surfaces. You can cut the rolls according to your preferred length without affecting the rest of the rolls.

This video provides more information on how to clean the flame sensor on the water heater.

6. Reinstall the Burner Assembly

Once your flame sensor is clean, you should screw it back on. Since you did not interfere with the entire burner assembly, you only need to ensure that you position the flame sensor correctly. 

Confirm that all the wires are connected as they should be before returning the burner assembly to the water heater. 

7. Confirm That the Water Heater Is Functioning Correctly

Switch on the water heater again and let it run for a few minutes. If everything is as it should be, screw back the cover plate. 

If the water heater runs and stops after some time, you probably have a deeper issue. The flame rod may be faulty, or there may be other issues with the water heater. 


Cleaning your water heater’s flame sensor is easy if you feel comfortable working on it. However, if you are worried about dealing with wires or don’t know how to cut off the gas supply, it is best to play it safe by hiring an A.O. Smith water heater professional.