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How to Tell If Your Flame Sensor Is Bad?

If you’re here, you’ve likely asked yourself “How do I know if my furnace flame sensor is bad?” It’s a key thing to figure out if you want your furnace to operate adequately and efficiently. There are several key signs that your furnace’s flame sensor is not working right, particularly if the furnace’s cycle is inexplicably short or if there’s damage on the sensor itself. But let’s go into a bit more detail.

What is a flame sensor and why does it matter?

The flame sensor’s purpose is self-explanatory – it’s meant to sense when there’s a flame in the furnace and when there isn’t. If the sensor doesn’t notice any fire, the gas valve will close as to not leak any unnecessary gas. Another way a bad flame sensor can impact your furnace is that it may not sense the burning hot fire and shut down the furnace even though it’s working well.

How to tell if a flame sensor is bad?

There are several main signs that can point to a bad flame sensor:

  • Cracked porcelain on the flame sensor.
  • Corrosion and/or soot on the flame sensor.
  • The furnace shuts down several seconds after lighting up.

Ideally, you’ll notice the problem through the first two signs and won’t have to get to the third one. Or rather, you’ll maintain your furnace and flame sensor well enough so that they don’t even get to these first two signs either. However, if the sensor has gotten so bad that it shuts your furnace in mere seconds, you definitely need to act.

What causes a flame sensor to go bad?

The most common problem for a flame sensor is soot buildup. They are, after all, exposed to frequent fire and smoke, as well as other debris.

Rust and corrosion are other very common problems for flame sensors and they too are caused by infrequent cleaning and maintenance. At the end of the day, the flame sensor is the front part of a low power electric circuit so it can suffer from the same problems all electric circuits can suffer from when it gets corroded. If the problem isn’t addressed, cracks in the porcelain can become noticeable very soon, and after that come the imminent shutdowns.

How to locate and clean the flame sensor?

Every furnace is different so you’ll want to consult with your furnace’s manual. Most of the time you’ll have to unscrew and remove the furnace’s cover. Then, just look outside the furnace’s burner – the flame sensor will be made of metal and typically has white, yellow, or brown porcelain around its end to protect it from getting in contact with the furnace’s metal.

As for cleaning the sensor, that’s easy – just remember to shut off the power and gas of the furnace, then remove the sensor, clean it with an emery cloth, and reinstall it back into its place.

Replacing the flame sensor is just as easy, you’ll just need to get a new one that’d work with your furnace.

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