Flame Sensor: A Crucial Part of a Furnace

Flame sensor in a furnace

The Flame Sensor

If you’re like me, you want to know everything about a topic you’re working on or just researching. The furnace comes with many components to analyze.

Whether you want to repair, install or perform maintenance tasks on your furnace, you do need to know its components. One of those components is the flame sensor.

How to break it down?

Although the word flame goes hand in hand with furnace, the word sensor may throw you off a bit. I can simplify this a bit for you.

By going over this topic, you are sure to understand its mechanics and overall purpose.

What is the Flame Sensor on a Furnace?

As with any major appliance, safety features are always important. The same thing goes for a furnace.

The flame sensor is an essential part of the furnace’s safety system. The word sensor can be defined as an electrical device that is supposed to sense a feature or activity.

Measuring the flame

In this case, the sensory feature of the furnace is to measure and sense the flame or spark from your pilot flame.

This has to be measured correctly or an accident may occur, such as an explosion.

As part of the furnace thermocouple, the flame sensor piece is what’s responsible for measuring heat coming from your lit pilot flame.

How Does a Flame Sensor Work?

The way it works is simple. The whole process is an ignition sequence. Think of it as a chain reaction

It begins as the igniter or spark ignites the gas to create the flame. The flame sensor’s job is to create a current of electricity.

One note on safety:

This current has to be on the proper level to function. The furnace will come with a control board that measures this reading once the ignition process starts.

If the control board does not read the correct level of electrical current, the furnace will cut off the fuel intake to prevent an explosion or fire.

This is what makes the flame sensor so important. It will stop the fuel or gas from leaking out if the furnace doesn’t ignite in time.

This could be the problem:

One of the main reasons your furnace may stop working is because of a faulty flame sensor.

Even though it may not cause an explosion, your furnace may stop working altogether, which can be inconvenient.

An uncomfortable winter…

I’ve had my heater quit on me in the middle of winter, and it was not fun. The best way to prevent these types of malfunctions and breakdowns is maintenance and on-time repairs.

Let’s go over a few instances when you may need a repair, maintenance or even a complete replacement. 

Replacement Furnace Flame Sensor by Goodman

Typical flame sensor replacement part

A Malfunctioning Flame Sensor is NOT Fun

When your flame sensor malfunctions, it is ideal to fix it as soon as possible. You may find many reasons why a flame sensor is not working.

Keep it clean:

The first reason can be a dirty flame sensor. As with any mechanical part, you should always keep it clean for optimal performance.

You can keep your sensor clean by maintaining it on a regular basis.

A sensitive sensor:

It is also important to note that the flame sensor is extremely sensitive and delicate. Therefore, any type of dust could affect its performance.

A bad sensor:

Carbon buildup can also cause a faulty sensor. However, if you always keep your flame sensor clean, it could just be that the sensor has gone bad. 

Replacing it:

In this case, you can try to replace the flame sensor yourself. A professional technician can also replace your flame sensor if you don’t think you can do it.

As a safety precaution, you can replace and install a new flame sensor every two to three years. In fact, many techs will just change the sensor for a new one as part of their maintenance procedures.

A critical symptom:

One symptom of a bad flame sensors is the ignition, which has already been discussed. If the furnace does light but shuts down immediately, this would indicate a flame sensor issue.

Make sure that you also look at the porcelain on the sensor. If it is cracked, you could be dealing with a broken flame sensor.

You may also notice a sensor filled with corrosion and soot.

How to Clean a Flame Sensor

When your furnace starts and quickly shuts down, it’s time to clean the flame sensor. The first thing you should check that you have is the necessary equipment.

Have the right tools:

This will include the proper tools. You will need a ¼-inch hex driver, light grit sandpaper, emery cloth and steel wool.

Make sure that you also have extra paper towels on hand. Once you have the proper tools, you can start the cleaning process.

Turning off the power:

The first step is to shut off all the power going to the furnace. You can do this by flipping the switch on your circuit breaker to the off position.

I always recommend you do this when repairing, installing or maintaining your home appliances. If you feel safer, you can also shut the power off on the furnace itself.

Shut it off at the furnace:

This toggle switch should be mounted on or somewhere near the furnace. Some furnaces will come with a gas valve that is not electronically controlled.

You can check this by referring to the manufacturer’s instructions. If this is the case, you will need to shut off the gas going to the furnace.

Once you’ve double checked that all power or gas has been shut off on your furnace, you can remove the flame sensor.

Sliding the sensor out:

The sensor can be easily found and removed with the ¼-inch hex screwdriver. The sensor should slide out where you can carefully remove it.

Time to clean:

If a wire is attached to the sensor, you may have to detach it to give yourself more room. Gently clean and rub the metal rod with the sandpaper.

Your main goal is just to clean off any buildup. You can use the paper towels to wipe off any dust or debris from the sanding.

Putting it back:

This should complete the process of cleaning the sensor. Next, simply replace the flame sensor back in its place. 

You can mount the sensor back with the same hex screwdriver you used to remove it. Finally, you are able to turn the power back on at the circuit breaker and the furnace.

How to Test a Flame Sensor

If you cleaned your flame sensor but it’s still malfunctioning, you may need to replace it. You can check this by performing a test on the flame sensor.

Use a multimeter:

The good thing about this step is that it’s easy to do. All you need is a multimeter and a light. You will have to find the flame sensor to perform the test.

Finding the flame sensor:

Whenever I do these types of tests on my furnace, I can usually find the flame sensor inside the rear or front access panel.

There’s not much to this component – you’ll see a thin, rounded rod about the diameter of a coat hanger near your row of burners. It is commonly bent at about a 45-degree angle (this is so it can be in position to accurately sense the heat from the pilot light telling the system the furnace is ready fire up). 

A friendly reminder…

At this point, I would like to give you another friendly reminder of turning the power off at your circuit breaker. You can also unplug the furnace as well.

Here’s what to do:

Next, you’ll need to loosen the clamp that secures the sensor in place. You can use a Phillips screwdriver for this entire part.

Setting the multimeter:

Multimeter to test flame sensor

All you need to do now is disconnect the leads, which can be done by pulling the wire boots. Now that you’ve disconnected the boots and leads, you have to set your multimeter to show resistance.

A low resistance:

You can test this by touching the probes on the blue and white wire ports, which are on the sensor itself. You should be getting a low resistance reading on your multimeter.

A high resistance:

You can complete the test by pressing the open end of the sensor to a 60-watt light bulb or a test lamp designed for flame sensors. The multimeter should be giving you a high resistance reading.

No discrepancies:

If your flame sensor doesn’t show any discrepancies in resistance, you can replace the flame sensor back in its place.

Testing if a flame sensor is bad

FAQs about Flame Sensors

Q: Why do flame sensors fail?

A: This can be due to a few factors. However, the most common issue is a dirty flame sensor. Another reason can be due to age and usage of the sensor.

Q: What does a flame sensor do in a furnace?

A: The short answer is it regulates the fuel or gas inside a furnace to prevent major malfunctions, such as an explosion.

Q: What composes a flame sensor? 

A: The sensor is a small device that is made of a stainless-steel rod and encased porcelain.

Q: How do you clean a flame sensor?

A: The flame sensor can be easy to clean if you follow the steps carefully. Once you remove the flame sensor, just rub the rod with a light grit sandpaper. You can use a dry paper towel to wipe off any dust and debris.

Q: How much do flame sensors cost?

A: Now we’re getting to the good stuff. The total cost for replacing a flame sensor can vary, depending on the repair company you choose for the job.

The price ranges can be anywhere from $350 to as much as $500. This can include the cost of travel and the diagnostics charge.

Q: What are the signs of a bad sensor?

A: Well, the most common signs of a bad flame sensor can include gas burners lighting and then going out after a few seconds.

Another sign is soot that covers the tip of the sensor. You may also find the white porcelain of the flame sensor cracked or broken.

Overall Takeaway

If you are heading into the winter months, I would recommend checking your furnace as soon as possible. This would include the flame sensor.

Check it before winter:

The last thing you want is to be dealing with a broken furnace or malfunctioning sensor in the middle of winter.

Doing the work:

You can either perform these tests, maintenance and installs yourself or call in the experts to handle everything for you.

Be prepared:

Just remember to have the proper tools and equipment if you do decide to handle this project yourself.

Your main goal:

In the end, your main goal is to have your home nice and warm. Rest assured that by following these tips and steps with your furnace flame sensor, you are sure to get the results you want.

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