Furnace Flame Sensor – Troubleshooting Guide

A flame sensor is one of the parts of the furnace that helps make sure that the unit is safe to use.

The main job of the sensor is to detect the flame; in case that does not happen, the sensor is going to shut the furnace down. Usually, these parts have to be replaced every 5 years and cleaned once a year.

What Does the Furnace Flame Sensor Do?

A flame sensor is an extremely important safety device. It is there to detect whether or not there is a flame burning inside the burner assembly.

If there is no flame, the sensor is going to immediately shut the furnace off to avoid a gas leak that can be dangerous. Basically, the component’s job is to prevent explosions and carbon monoxide poisoning.

Only gas furnaces can have such a sensor. 

They are usually anywhere from 2 to 4 inches and are mounted close to the burner (the metal rod has to be positioned in the flame). Some sensors are straight, while others have an ‘L’ shape.

How does a flame sensor work?

The actual sensor is a thin metal rod that can create an electric current that is going to confirm that there is flame present in the furnace.

The combustion process has to begin once the gas valve opens. However, if the sensor couldn’t detect a flame within 10 seconds, it is going to shut the system down.

This cycle will be attempted two more times, but if the gas valve continues to open and there is still no flame, the sensor is going to lock the system out.

In such a case you would have to manually reset the system or call an HVAC expert to do that. The latter might be a better decision as there may be a serious problem with your gas furnace.

Read: HVAC UV Light: Everything You Need To Know

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What Are the Signs that My Furnace Flame Sensor Is Bad?

There are a few ‘symptoms’ of a bad flame sensor.

Do bear in mind that, at times, you can easily fix the problem by simply cleaning the sensor.

The furnace turns on and then shuts down in 2-30 seconds

If the part is faulty, it won’t be able to detect any heat and, as a result, it is going to send a signal that will close the gas valve. Of course, in such a case, the furnace is going to shut down as there is no gas.

Such ‘short cycling’ can happen three times before the sensor locks the system out.

The porcelain on the sensor is cracked

Short cycling might also be caused by a cracked flame sensor. However, at times, the element can continue to operate even with a damaged surface (though its ability to detect heat will be limited).

The outer layer of a flame sensor is made out of porcelain and that is the exact layer that is responsible for heat detection.

There is corrosion or dirt on the flame sensor

A dirty or corroded sensor may not be able to detect the heat. In such a case (you guessed it), short cycling will occur.

Remember that the sensor is basically located right in the flame, it is also constantly dealing with a low-power electric circuit. So, corrosion and soot buildup are relatively common and normal.

Read: American Standard Furnace – Troubleshooting Guide

How do you test a flame sensor?

Usually, homeowners would invite an expert inspect the flame sensor. However, if you feel comfortable working with electricity and have the necessary equipment, then you can attempt to test the flame sensor on your own. 

  • Switch the furnace’s power off and close the gas valve.
  • You are going to need a wrench or a driver for the screw that is holding the element in place.
  • Carefully pull the flame sensor out.
  • Inspect the element – if it’s cracked or is missing pieces, then it would have to be replaced.
  • If you can’t really tell, whether the flame sensor has gone bad or not simply by looking at it, then use a multimeter.
  • The multimeter has to show low resistance when you touch the probes to the blue and white wire ports.
  • Press the flame sensor’s open end to a 60-watt bulb or a test lamp. Now, the multimeter has to show high resistance.
  • If the device didn’t show different readings, then the flame sensor would have to be replaced.

How Long Do Furnace Flame Sensors Last?

A flame sensor will wear out over time and that is completely normal.

In fact, you would have to get a new sensor at least a few times in the lifespan of your furnace, as flame sensors last for about 5 years, while a furnace can serve you for around 15 years or more.

You can extend the lifespan of this important element if you regularly clean it and manage to check its state on a regular basis. 

Read: How Long Do Flame Sensors Last?

Can You Clean a Flame Sensor?

You can certainly clean your flame sensor. In fact, such a simple manipulation is going to prolong the lifespan of your furnace, so make sure to follow these steps every once in a while.

  1. Completely shut the furnace down and close the gas valve.
  2. Open the combustion chamber door and take out the mounting screw that is holding the flame sensor in place with a wrench or driver.
  3. Gently pull the sensor out (be careful as the element might end up sliding out).
  4. Inspect the sensor – if it looks sooty, slightly corroded, or burnt, then it does require cleaning.
  5. Use steel wool, emery cloth, or light-grid sandpaper to gently rub the sensor.
  6. After that, wipe down the metal rod (you can use a paper towel).
  7. Check the furnace’s pilot and ignitor and blow off any accumulated dust.
  8. Reattach the flame sensor, close the combustion door, and turn the furnace back on.

How often to clean furnace flame sensor?

A flame sensor can stop properly functioning if there is the slightest carbon coating on the element.

This might happen quite often, taking into consideration that the majority of gas furnaces are located in particularly dusty places like basements, for example. The airborne particles of dust will stick to the sensor and then get burnt which, in its turn, causes carbon buildup.

Experts recommend getting your furnace tuned up once a year. That is when the professionals are going to clean the flame sensor.

However, if you want, you can do this manipulation a bit more often. Simply check the sensor every few months and if you see any buildup on its surface, then gently clean the little helper.

Read: Why Is My Furnace Beeping?

When Should I Replace My Flame Sensor?

You should replace the sensor if it has been physically damaged. For example, if you see cracks or missing parts during your inspection, then the element would certainly have to be replaced.

If the sensor looks fine, but it doesn’t show any signs of ‘life’ when you use a multimeter, then you would have to get a new one.

How much is a new flame sensor for furnace?

Actual flame sensors are not very expensive. The majority of them cost less than $40.

However, if you decide to invite a furnace repair company for that matter, the experts might end up charging you $350-$500. This price includes the cost of the new part, the travel expenses, the diagnosis, and the cleaning.

If you decide to replace the part yourself, then make sure to take the old flame sensor with you to a local furnace parts supplier. Don’t forget to write down the brand of the furnace, the serial number, and the model number.

To replace the sensor, you would have to:

  • Shut down the furnace and the gas valve.
  • Remove the screw and pull the sensor out.
  • Detach the wire from the flame sensor.
  • Push the new part into the opening, secure it with a screw, and attach the wire.