What To Do With A Dirty Flame Sensor

It can be scary when on a cold night your furnace starts to cycle on and off, or only comes on for a few seconds and goes off again. Before you start to worry about the cost of a furnace replacement or repair, check to see if the flame sensor is dirty. This is a common issue and one that is easy to diagnose.

A Home High Efficiency Furnace With A Residential Gas Water Heat
A home high efficiency furnace with a residential gas water heater & humidifier.

If your furnace has been cycling on and off, or not staying on for very long, the problem could be a dirty flame sensor. If you notice that your furnace is cycling on and off or struggling to stay on, the team at DeZiel Heating & AC can help. Call us today.

What is a Furnace Flame Sensor?

A flame sensor is a small metallic rod that sits in the burner assembly of your furnace. It is often bent and located in front of the pilot flame. The job of the flame sensor is to tell the furnace whether or not there is a fire burning in the burner assembly.

The flame sensor is a safety device that is there to shut off the furnace if it does not detect a fire burning in the burner assembly. Without it, your pilot light could go out and you would be none the wiser, leaving your home filled with deadly carbon monoxide.

Sadly, if the flame is present but the sensor can’t detect it for whatever reason, you could end up without heating for at least an hour. This is because if the flame sensor doesn’t detect a flame, it automatically shuts off the furnace to prevent a gas leak. In most modern units, after three shutdowns, the furnace will enter a lockout mode and stay off for an hour before trying to turn on again.

Flame Sensor vs. Thermocouple

Although you may have heard people use the terms interchangeably, a flame sensor is not the same thing as a thermocouple. In fact, they serve two different purposes.

A thermocouple is located next to the pilot light and its job is to shut off the gas to the burner assembly if the pilot light goes out. So, if your pilot light goes out, your furnace will shut off thanks to the thermocouple. It is found on standing pilot furnaces with pilot lights.

On the other hand, the job of the flame sensor is to make sure that there is an actual fire burning in the burner assembly. If it does not sense a flame, it will also shut off the gas to prevent a gas leak.

A Dirty Flame Sensor

Over time, because your flame sensor is surrounded by flame, it gets covered in carbon deposits, dust, and other debris. When this happens, it can become difficult for the sensor to detect the presence of a flame.

If your furnace is having trouble staying on or keeps cycling on and off, you should check your flame sensor to see if it needs to be cleaned. Luckily, this is a pretty easy process that you can do yourself.

Signs of a Bad Flame Sensor

It can be easy to tell when your sensor is dirty, corroded, or faulty. The first sign is usually that your gas burner lights up and then quickly goes out again. If this is happening, it’s a good idea to check your flame sensor.

Another sign that your flame sensor needs to be cleaned is if you spot soot all over the tip. It’s time for professional service if the tip is all black. However, if you look at the sensor casing and spot cracked porcelain, then it’s time to replace your flame sensor entirely. The porcelain is there as an insulator to protect the sensor from the heat of the flame and, once it’s cracked, it can no longer do its job.

Cleaning Your Furnace Flame Sensor

If you think your flame sensor is just dirty, then you can try cleaning it yourself. If you don’t feel up to it or don’t want to risk damaging your sensor, then you can always call a professional.

The first thing you need to do is shut off the power to your furnace at the breaker box. Once that’s done, you can remove the sensor, clean it up, put it back in place, and then test it out to see if it’s working properly. If the sensor is still not working after you’ve cleaned it, then you may need to replace it entirely. Here are the steps in detail.

Turn Off The Furnace

Like every other home appliance, it’s always best to disconnect the power before working on your furnace. This will prevent you from getting an electric shock or damaging any of the sensitive electrical components in your furnace. Since furnaces have gas lines, it’s doubly important to make sure the power is off before you start working to avoid burns or the risk of fire.

If your furnace has a switch that looks like a light switch, put it off there. If it doesn’t, then go to your breaker box and switch off the circuit breaker for your furnace. If you’re not sure which one it is, then you can either look for a label or turn off all the breakers until the power to your furnace is cut. If you have a furnace that does not make use of electronic control, you’ll have to shut off the gas valve to the unit as well. If you feel all of this is too much for you, please consult with a professional.

Remove The Old Flame Sensor

Now that the power is off, you can remove the flame sensor. The first thing you need to do is identify where it’s located. In most furnaces, this will be located on or near the burner assembly. It is usually held in place by a screw which you can remove easily.

Clean The Sensor

To clean the flame sensor, you’ll need a wire brush or a light grain sandpaper. Use it to gently remove any debris, carbon deposits, or corrosion that has built up on the metal rod. Once you’re done, you can use a paper towel or soft cloth to wipe it down and remove any residual dirt.

Reattach The Sensor And Test

Now that your sensor is clean, you can put it back in place and screw it in. Be careful not to overtighten the screw as that can damage the sensor. Once it’s in place, you can turn on the power to your furnace and test it out. If it still isn’t working properly, then you may need to replace your flame sensor entirely.

How to Replace a Broken Sensor

To replace a broken sensor involves the same steps as cleaning a dirty one. The only difference is that, once you’ve removed the old sensor, you’ll need to purchase a new one and install it in its place. When you’re choosing a replacement sensor, make sure that you get one that is compatible with your furnace model. To be double sure of the diagnosis, it is best to ask a professional.

Get Professional Help 

If you don’t feel comfortable working on your furnace, then you can always call a professional at DeZiel Heating & AC. We will be able to quickly diagnose the problem and get your furnace up and running in no time. Not only that, but our team can also perform regular maintenance to prevent future problems from occurring. This will help prolong the lifespan of your furnace. Contact us today at 763-684-3965 to get started.


And We'll Come Running to Your Rescue.

DeZiel Heating & Air, Inc Event