How To Tell If Furnace Flame Sensor Is Bad? (Step-By-Step Guide)


Having a bad furnace flame sensor can be quite dangerous. Without a properly functioning flame sensor, natural gas or propane can leak into your home and be accidentally ignited. That’s why it is very important to know how to tell if a flame sensor is bad.

Flame sensor troubleshooting is usually done by an HVAC expert during regular furnace maintenance. What you can do on your own is look for symptoms of a bad flame sensor if you suspect something might be wrong.

Note: According to HVAC.com, the expected lifespan of a flame sensor is about 5 years. Most furnaces last anywhere from 10 to 20 years. That means that you will have to replace a flame sensor 1x, 2x, or even 3x during a furnace’s lifespan. Having a bad flame sensor is completely normal; you just have to know how to detect that it’s bad and know how to replace it.

We will cover the 3 most common symptoms of a bad furnace flame sensor one by one. First of all, however, we need to look into what exactly a flame sensor is and how does a flame sensor is in order to be competent enough to diagnose a sensor failure.

furnace flame sensor
Here is a standard Carrier flame sensor. You can see all the parts (wire, screw, rod, porcelain) that we’re going to refer to in this article.

After that, we will also explain (in easy-to-follow step-by-step guides):

  • Where in the furnace is the flame sensor located? If you truly want to do a DIY diagnosis of your sensor, you will need to find it. It might be just dirty (proceed to cleaning) or cracked (proceed to replacement).
  • How to clean a flame sensor? Cleaning the sensor is a part of regular maintenance and can prevent the flame sensor from breaking down in the future.
  • How long do flame sensors last? If you have an old furnace, changes of flame sensor misbehaving increase exponentially. Flame sensors last for about 5 years, furnaces last for about 15 years. You will, on average, have to make 2 flame sensor replacements within the lifespan of a furnace.
  • How to replace a flame sensor? You can DIY flame sensor replacement if you have some technical skills.
  • How much does a flame sensor replacement costs? If you do it yourself, it’s about $100 cheaper than professional taking care of the sensor replacement.
  • Finally: Furnace flame sensor troubleshooting. A simple 5 step-by-step procedure how if tell if a new or properly cleaned flame sensor of operating as it should again.

What Is A Flame Sensor? How Does A Flame Sensor Work?

The flame sensor is a part of all furnaces and it tells if the furnace has fired correctly. It facilitates the communication between the gas valve and furnace firing. Underneath the cover, this is a basic heat thermocouple sensor; the flame sensor responds to changes in temperature.

It’s important to understand here that a gas valve (when open, it allows for the flow of natural gas or propane) and the furnace firing are two separate systems. The only thing that tells the gas valve if it needs to stay open and allow for the flow of gas is the flame sensor.

If there is something wrong with the flame sensor, the gas valve might stay open even if the furnace is not combusting the gas. That will create a gas leak (since the gas is not being used as fuel) and can potentially lead to an explosion. That’s why it’s crucial to be able to tell if the flame sensor is misfunctioning.

Here is the step-by-step way of how does a flame sensor work when functioning properly:

  1. Turn on the furnace. The gas valve opens to allow for the flow of gas.
  2. Furnace pilot light ignites the gas in the combustion chamber and the flame sensor detects the heat.
  3. Once that heat is gone (furnace doesn’t need to combust gas anymore), the flame sensor detects the drop in temperature and shuts down the gas valve.

This shutting down the gas valve when the combustion cycle is over is the main purpose of a furnace flame sensor.

Here’s what a bad flame sensor does: Even when the combustion is undergoing, the faulty flame sensor won’t detect the heat. Believing there is no furnace firing happening, it will shut down the furnace immediately (even during a proper cycle).

This gives us an insight into the most common way of how to tell if the flame sensor is bad:

3 Signs Of A Bad Flame Sensor

Now, it’s not all that hard or expensive to replace a faulty flame sensor. The biggest challenge is how to detect that your flame sensor is, in fact, not functioning as it should.

Note: One of the most tricky things is to distinguish between a faulty flame sensor and just a dirty flame sensor. To determine that, you will need to open your furnace and locate the sensor (you will find step-by-step instructions further on) and we will also explain how to clean a furnace flame sensor.

Here are the 3 most common ways how to tell if you have a bad flame sensor:

  1. Furnace turns on and off down very quickly (after 2- 30 seconds). This is the classic symptom of a bad flame sensor. A faulty sensor doesn’t detect the heat that turning on a furnace makes, and subsequently closes the gas valve, shutting down the furnace. We see this as furnace short cycling in a few seconds. Note: Can a bad thermostat cause a furnace to short cycle? That is possible but in very limited cases. When the furnace runs then shuts off and starts again, the out-of-order flame sensor is usually the culprit.
  2. Flame sensor is cracked. You can notice short cycles, but to detect a crack in the flame sensor, you have to locate the sensor first (we’ll show you how to do that later on). A bad flame sensor will have a crack in the outer porcelain part. That part is responsible for heat detection (temperature changes). If it’s broken, its ability to detect heat will be very limited or completely diminished.
  3. Corrosion or dirt visible on the flame sensor. Anything that limits the heat exchange on a sensor will lead to unusual consequences. In the case of flame sensors, you will see either corrosion or dirt (soot) on the sensor itself. That may be a result of wear and tear, too-high humidity levels, or irregular or even non-existent flame furnace cleaning. With the full ability to detect heat due to a layer of corrosion or dirt on the sensor, you will often see your furnace short cycling.

As you can see, the easiest way to tell if your furnace flame sensor is out of order is to detect short cycling. Anybody can notice a furnace turning on and off in rapid succession.

Once you see something like this happening, you have to locate the flame sensor to check if:

  • Flame sensor is broken (porcelain is cracked). You will need a replacement; we explain how to replace a flame sensor further on.
  • Flame sensor is just dirty (corrosion or soot accumulation). You will need to clean the flame sensor; we also explain that further on.

First, however, you will need to open the furnace and locate the flame sensor. Here is how to do it correctly:

Where Is The Flame Sensor Located? (How To Find It Yourself?)

In order to have a look at a flame sensor, you will have to first take the furnace cover off. This is a big sheet that is usually held in place by screws, tabs, knobs, or what have you.

The sensor is located directly below this furnace cover. However, there are several furnace parts there. You have to figure out which part is the sensor.

How does a furnace flame sensor look like?

The flame sensor looks like a few-inch-long metal rod. It is mounted on the burner assembly, and you will be able to distinguish it from other parts by checking the white or yellow porcelain. Shape-wise, most flame sensors are straight. Some sensors might also have a 45-degree or even 90-degree near the end.

You will also notice the flame sensor is wired by a single wire. This wire relays the signals from the sensor. In order for the sensor to actually detect heat, a part of it is pushed into the combustion chamber; that’s where the heat is generated.

Here is a safe way how you can take the flame sensor out and look for cracks/dirt:

  1. Switch off the power to your furnace. This is a safety measure.
  2. Close the gas valve (if it’s open).
  3. Take the furnace cover off and locate the sensor.
  4. Flame sensor is usually mounted on the burner assembly by a single screw. Unscrew it.
  5. Pull the sensor out.

When you have a sensor in your hands, you can safely diagnose what the trouble with the flame sensor might be. There are two options:

  1. Sensor is just dirty. You might notice signs of corrosion or soot accumulation on the part that protrudes into the combustion chamber. You have to proceed to clean it as described in a chapter below this one.
  2. Sensor is cracked. You might see a crack in the porcelain or in the insulation around the sensor. In this case, you will have to replace the sensor with a new flame sensor; just follow the steps followed below (you might need professional help as well).

Let’s first look at how to clean a dirty sensor. After that, we will look into how to replace a cracked sensor as well.

How To Clean A Flame Sensor On Furnace In 5 Steps

If your furnace is misbehaving due to a dirty flame sensor, you only need to clean the sensor and plug it back in.

Cleaning the flame sensor is pretty straightforward, with one key part. That includes what kind of cleaning tool you use to brush the corrosion or soot buildup on the flame sensor.

Here is the simple 5 step procedure how to clean a furnace flame sensor:

  1. Shut off all the power. This is a safety precaution.
  2. Remove the furnace cover, locate the sensor, and take it out. You can use a 1/4 hex screw to unscrew the single mounting screw that attaches the flame sensor to the burner assembly.
  3. Clean the sensor. This is the crucial part; for corrosion, you can use grain sandpaper, wire brush, or steel wool. Once the corrosion is removed, use a dry cloth to remove all the remaining pollutants. If your sensor is just dirty (soot buildup), you should use emery cloth. This is a coated abrasive that removed hardened carbon (soot) extremely well.
  4. Screw the sensor back in, and reinstall the furnace cover.
  5. Restart the furnace.

If the soot or corrosion was the culprit, the furnace should be working without problems. If, however, you still experience signs of a bad flame sensor such as furnace short cycling, chances are that the flame sensor is broken and needs to be replaced.

How To Replace A Flame Sensor On Furnace (Thermocouple Replacement)

If you see a crack on the porcelain part, insulation crack, or you have cleaned the sensor and you’re still experiencing bad flame sensor symptoms, it’s time to replace a flame sensor.

As mentioned in the introduction, a flame sensor lasts for about 5 years. It is completely normal to replace a sensor every 5 years or so.

Can you replace a flame sensor yourself? 

Sure you can. You can seek out professional help if you’re not a technical type of person. There is only one tricky part to furnace flame sensor replacement; that’s detaching and reattaching the single wire that connects the sensor to the furnace.

Here are 7 simple to follow steps how to replace a flame sensor on a furnace:

  1. Shut down the furnace and cut the power.
  2. Remove the furnace cover, locate the sensor, and remove that 1 mounting screw.
  3. Here is the tricky part: A single wire connects the sensor to the furnace. You have to carefully detach that wire from the flame sensor.
  4. Get a new flame sensor and place it in the same position the old flame sensor was in.
  5. Screw in the mounting screw to mount the flame sensor. It should be stable.
  6. Reconnect the new flame sensor to the wire you detached from the old sensor.
  7. Place back the furnace cover, screw it in, and restart the furnace.

This is not rocket science; everyone should be able to replace an old flame sensor with a new one on a furnace.

When deciding if you should be calling a professional to replace a bad flame sensor, you should consider the costs:

How Much Does A Flame Sensor Replacement Costs?

A new flame sensor usually costs less than $50. For some furnaces, you will be able to get a $20 flame sensor.

Here is how you should would the replacements costs if you a DIY replacement or call a professional:

  • DIY replacement: Up to $50 for sensor + 20 minutes of your time.
  • Professional replacement: Up to $50 for sensor (parts) + About $100 for service fee (labor).

At the end of the day, you’re looking at $50 vs. $150. Do keep in mind that within the average lifespan of your furnace, you will have to replace the flame sensor 2 times. So, that’s a $200 service fee that you don’t have to pay if you know how to replace the flame sensor yourself.

The biggest reason why more people are not replacing their own sensors is not a difficult replacement procedure. It’s the ‘which flame sensor do I need part’.

You have furnace-specific and universal furnace flame sensors. The best advice is to pull the old sensor out of your furnace, go down to your local hardware store, and ask the guys there to give you the same one or a universal one that will work with your furnace.

When you do replace the flame sensor (or clean it), you have to troubleshoot it to check if it’s working correctly.

Furnace Flame Sensor Troubleshooting (Last Step In All Cases)

It doesn’t matter if you replaced, cleaned, or just moved the flame furnace; you always have to check if it’s working correctly. That means you have to do the flame sensor troubleshooting.

This is dead simple. Here is how easy it is to troubleshoot a flame sensor on the furnace:

  1. Reattach the cleaned or new flame sensor, screw on the furnace cover, and you are ready to go.
  2. Switch the power back on and restart the furnace.
  3. If you don’t experience short-cycling as you did before, the flame sensor is performing its job.

If even after replacing a flame sensor you experience the same problems as before, the root cause of your furnace issue might not be a bad flame sensor. In this case, it’s best to consult with an HVAC professional to make a full diagnosis of what’s wrong with your furnace.

We hope this was helpful. If you have any questions, you may pose them in the comments below and we’ll try to help you out as best we can.

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