How To Clean The Flame Sensor On A Furnace (Do This!)

Ryan Womeldorf
by Ryan Womeldorf

With so many components in your furnace, it only makes sense that one may stop working from time to time. More often than not, a dirty or clogged filter will be the cause of a furnace that isn’t working properly.

The flame sensor, meanwhile, can also stop working from time to time. With proper cleaning and maintenance, however, it should keep working optimally for a long time to come. The process of cleaning the flame sensor is easy. Simply shut off the power to the furnace, remove the flame sensor, clean it, and put it back. If the flame sensor is damaged, consider replacing it entirely.

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What Does the Flame Sensor Do?

The flame sensor, located in the burner assembly, is a pretty simple device. It is generally quite small and thin, a bent metallic rod that sits at the front of the flame stream on the inside of the furnace.

The flame sensor actually has a pretty basic purpose and that is to confirm that there is fire present when the gas valve is open.

  • Preventing gas buildup. Perhaps the most important role of the flame sensor is to ensure that the gas valve isn’t just open and pumping out gas. When that happens, there can be a dangerous buildup of gas within. So, when the furnace kicks on and the burners ignite, the flame sensor is only given a short window to look for the flame.
  • No flame detected. When the flame sensor doesn’t pick up a flame, it shuts the furnace down automatically. For most modern units, there are three allowable shutdowns before going into the “safety lockout” mode. That mode is meant to prevent consistent issues from occurring and usually requires a reset before the furnace will work again.
  • A dirty sensor. More often than not, the common culprit behind a malfunctioning flame sensor is that it has become dirty or clogged. This is a pretty simple fix but can lead to your furnace not working until that fix is implemented.

How Do Flame Sensors Get Dirty?

Keep in mind that flame sensors can simply go bad over time. The more likely explanation is that it is simply dirty as a result of carbon buildup. The flame sensor, being a smaller part, is particularly sensitive to variations in the readings that it takes.

Even a slight coating of carbon can cause the flame sensor to improperly read and then shut down. Since most furnaces are located in attics and basements, there tends to be a lot of dust in the air. Over time, that dust can permeate into the furnace, burn up, and cause carbon buildup on the flame sensor.

Step 1: Cut the Power

Whenever you are performing maintenance or repairs on your first, it is an absolute must that you shut down the power to the furnace. This is not the same as turning the thermostat off; the furnace will still be on.

  • On/Off switch. Depending on your furnace, there may simply be a toggle switch, similar to a light switch, mounted near or on the furnace itself. Simply flip the switch to the off position and move on to the next step.
  • Circuit breaker. If you can’t find the switch or would simply feel more confident cutting the power to the area entirely, go to the circuit breaker box. Flip the breaker to the furnace off to completely cut the power.
  • Gas valve. It isn’t always imperative to turn off the gas, but there is one exception. If your furnace doesn’t have an electronically controlled gas valve, shut the gas off to your furnace as well. It is just safer to not have to worry about a gas leak while instituting repairs or performing maintenance.
  • When in doubt, stop. If you have a difficult time locating the on/off switch, turning off the circuit-breaker, or cutting the gas to your furnace, don’t go further. Fixing a furnace can be easy but it can be dangerous depending on the circumstances. Don’t be afraid to call in an HVAC technician to look at the issue.

Step 2: Take Out the Flame Sensor

With the power (and gas, if needed) off, you are now ready to locate and remove the flame sensor. The sensor is pretty easily accessible and can be found mounted using a hex head screw. Remove that screw carefully.

When the screw has been removed, the sensor should simply slide out so that you can clean it easily. Make sure that you remove the sensor carefully as it is a sensitive component. There also may be a chance that you will have to detach a wire located at the end of the sensor in order to give yourself more room to work.

Step 3: Clean the Sensor

Now that the sensor has been removed and you have more room to work, gently clean the sensor. Use a light grit sandpaper or a gentle scrubbing brush to rub the metal rod. Do not rub anything else; you are just ridding the rod of carbon buildup.

Remember that you aren’t performing heavy sanding. You want to give just enough force to remove that carbon buildup. When you’re done sanding, take a paper towel and wipe away any of the dust that gets left behind from the sanding.

Do not use any chemicals here that could leave a residue or potentially damage the sensor. Cleaning it is simple and straightforward; anything more is unnecessary.

Step 4: Replace the Sensor

The sensor does not need to be sparkling clean. Just make sure that the carbon has been removed before returning the sensor back into the furnace. If there is a wire, reconnect it before remounting the sensor on the burner assembly.

Use the hex head screw from before to remount. If you’ve lost it, you should be able to find an adequate replacement at your local hardware store. Make sure that it is mounted securely, replace the furnace unit door, and then return power to the unit.

Step 5: Check Your Work

When everything has been screwed back into place and is back where you found it, it is time to check our work. Turn the gas and electricity back on if you have not and then fire up the furnace once again.

Should the furnace take a little longer than normal to start up, that’s fine. It can take the furnace a few extra seconds to start or for the fan to kick on and run. By shutting down the power, it can cause the unit to reset and then run through a couple of safety checks before working properly again.

Make sure that the furnace is running normally once again by setting a temperature just a degree higher. This way, the furnace will kick on, hit the temperature, and then shut off once again as it normally would.

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What Happens if the Flame Sensor is Broken?

In most cases, simply cleaning the flame sensor will be enough to get the job done. But what happens when you come upon a flame sensor that has been broken entirely? Well, the only thing to do is to pull it out and replace it.

  • Replacing the sensor. Simply follow the same instructions above with one catch. Instead of cleaning the old sensor, simply pop the new one in its place. If this doesn’t solve the issue, there is another issue, and may require a professional inspection to determine.
  • Buying a new sensor. Should you determine that the old flame sensor is broken, replacing it is easy. Before you can replace it, though, you will need to buy the replacement. Big box stores may have the part that you need but, generally speaking, most hardware stores won’t have one.

To get a new flame sensor, you may have to check with a local contractor supply store. Don’t throw the old flame sensor away before buying the new one as it can help you identify the kind of sensor that you need. Worst case, you can simply provide the make and model of your furnace to help identify the part that you will need.

Ryan Womeldorf
Ryan Womeldorf

Ryan Womeldorf has more than a decade of experience writing. He loves to blog about construction, plumbing, and other home topics. Ryan also loves hockey and a lifelong Buffalo sports fan.

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