How To Clean Flame Sensor On Furnace

Most often than not, if your furnace flame sensor experiences problems, it is not broken but just dirty. If you find that your furnace fires up but shuts down shortly after, the chances are you’ve got a dirty sensor. Cleaning your flame sensor is a simple task and can be done DIY.

About Furnace Flame Sensors

A flame sensor is one of the essential parts of a furnace. It is not only designed to make your furnace work but also to keep it safe. A furnace flame sensor works to regulate the flow of gas coming into your furnace and automatically shuts down the unit if the gas burners do not light to prevent gas buildup or, worse, gas poisoning or catastrophic explosion.

These automatic shutdowns are often done three times before a “safety lockout” for an hour is done to dissipate the initial gas buildup before it is safe to be turned on again.

The constant inability of your furnace to start will not only leave your home cold but can also cause major problems like reduced efficiency to some parts of your furnace, and worse, totally breaks your furnace unit down.

What Makes Flame Sensors Dirty?

Carbon Buildup: Furnaces are usually located in home areas where lots of dust is in the air. Since flame sensors are in flame most of the time, dust and other particles can easily burn and cause corrosion and carbon buildup over time. A dirty flame sensor can lead to sensing errors that dictate your furnace to shut down every time you turn it on.

A dirty flame sensor is easily detectable. The most common sign of a sensor issue is when your furnace gas burners light up then go out after a few seconds. However, when you see gunks/ discoloration at the sensor’s tip, it also calls for a clean-up.

Doing a regular check-up for dirt and carbon buildups on your furnace flame sensor can improve not only its efficiency but for your furnace’s as well.

Furnace Flame Sensor Location

Where is the flame sensor on a furnace? The flame sensor is usually located near the burner assembly in the furnace.

It is a thin metal rod that may be straight or bent around the tip, depending on the sensor type or brand. It is mounted with porcelain insulation on the other end.

Cleaning a Furnace Flame Sensor Step-by-step Guide

A dirty flame sensor is a very common issue every homeowner faces with their furnace. The good news is, it is not hard to maintain a flame sensor at all. It is a very easy job, and you can easily do it yourself with the help of some of the common repair tools you can easily find at your home.

Tools You Need

  • ¼” hex driver or wrench, but may vary depending on your mounting screw type
  • Small piece of light grit sandpaper, steel wool, or emery cloth
  • Paper towel

Shut the Furnace Off

Whenever you do any HVAC unit or part (or any appliance per se) repair or clean-up, you must ALWAYS turn the unit off.

You can switch it off through the toggle switch on/near your furnace or through its dedicated circuit breaker box in the absence of a switch button.

NOTE: Make sure to shut off MAIN POWER and not the thermostat alone.

Also, if your gas valve is not electrically controlled, shut off the gas unit as well.

Remove the Flame Sensor

Locate a thin metallic rod-like part, usually at the back of the furnace and in the path of the burner. 

The flame sensors are typically kept in place by one or two ¼” hex head screws. Simply take the screws out and carefully slide the sensors out from the burner bracket or assembly.

Clean the Sensor

For some units, you may have to detach the wires connecting to the sensor to clean it well. This action is unnecessary, but disconnecting the cables will give you ample space and flexibility to clean the sensor later.

To clean the sensor, simply rub the metal rod gently with light grit sandpaper, steel wool, or emery cloth to eliminate the carbon buildup.

Note: Never use sandpaper as it will damage your flame sensor’s rod.

After that, use a clean and dry paper towel to get the excess particles and dust from the metal rod.

Put the Sensor Back

If you detached wires leading to the flame sensor, just reconnect it. Then, mount the sensor back to the burner bracket or assembly by putting back the hex screw/s.

Check if the Furnace Now Works Properly

Furnace flame sensor test: Turn the furnace back on. Wait for a few seconds as the furnace may not immediately start and may still be resetting and doing some start-upstart-up checks. Upon start-up, your furnace unit must operate normally and stay on until the thermostat commands an automatic shut-off.

Make sure to leave your furnace on for even one cycle to ensure the problem does not persist.

Furnace Flame Sensor Replacement

If you followed the steps above and your furnace still shuts off abruptly after turning it on, then your furnace flame sensor may be faulty and needs to be replaced.

To replace your flame sensor, simply redo the steps above (skipping the cleaning part) and replace it with a new sensor. In replacing your flame sensor, make sure to purchase the correct part replacement.

Where To Buy Furnace Flame Sensor

Local HVAC Shops

Various local HVAC providers mostly have flame sensors available. They offer a comprehensive list of flame sensors that you can choose from.

You have to make sure to purchase the correct part for furnace replacement. If you’re not sure which part to get, check your unit’s information inside the access panel, and contact us with your brand, model, and serial number to suggest the best match for your system.

Local Supply Store

Should you choose to use a universal flame sensor as a replacement, they are widely available at Home Depot, Menards, or other building supply stores near your area.

How Much Does It Cost To Replace A Furnace Flame Sensor?

Homeowners typically pay between $100 to $250 to replace the furnace flame sensor. If the unit is still under warranty, you’ll only pay for labor cost, wich is around $75 – $150.

Depending on your unit and model, a new furnace flame sensor will cost you between $10 to $100.

If you have problems with a dirty furnace flame sensor, call a professional HVAC technician for help. They can take care of all of your flame sensor issues and any other furnace problems you might have.

About Mas Broto

Have been in the heating and air conditioning (HVAC) industry for over 20 years. He is person that will grow and thrive to learn more about the HVAC industry throughout his career. Mas Broto is also a blogger, who's dedicated to bringing you the best knowledge to get ahead in the game of life.