Why Does My Furnace Keep Turning On & Off? | How to Clean or Replace a Flame Sensor


Have you noticed that your furnace has been turning on and off more frequently than normal lately?

The average furnace cycles on and off 3-6 times an hour.  When your furnace turns on and off more frequently, this is known as short cycling.  This is an issue many people are faced with at some time or another and is usually due to a malfunctioning Flame Sensor.  Unlike most other furnace repairs that need to be done by an HVAC technician, cleaning or replacing the flame sensor is something that you can typically do yourself.

So, what is a flame sensor?  The flame sensor is a safety feature built into your unit that detects if a flame is actually present and shuts down the unit if one is not.  This helps prevent the unit from continuing to release gas when there is nothing to ignite it.


  Rheem Flame Sensor – Part #62-23543-01

If the flame sensor has become corroded, it won’t be able to properly detect whether the unit has lit.  In this case, it will signal to shut of the gas causing your short cycling issue.

WARNING: Before starting any repair, always make sure that the appliance is completely disconnected from the power source and that the gas is shut off.

As always, this is meant to be a guide.  Especially when dealing with a heating unit, always consult the owner’s manual and do not hesitate to contact an HVAC technician if you ever unsure about a repair.

Tools Needed:

  • Screwdriver
  • Emery Cloth, Steel Wool, or Fine Grain Sandpaper
  • Clean Cloth or Paper Towel


How To Clean/Replace the Flame Sensor:

1. Shut off the Unit & Let It Cool

490451_mBefore working on the unit, make sure that the unit is completely disconnected from the power source and that the gas to the unit is shut off.  Depending on your model, you may be able to access a toggle switch to shut off the power or do so by accessing your breaker panel.  Turning the temperature down on the thermostat is not the same as shutting off the unit.

Once you have ensured that the unit is completely shut off, let it cool for at least 30 minutes before attempting any sort of cleaning or repair.

2. Locate the Flame Sensor

To access the Flame Sensor, remove the outer panel and set aside.  Look for an “L” shaped object near the pilot burner, this will be the flame sensor.  In most models, the sensor will be held in place by a single screw.  Remove this screw and slide the sensor out to more easily access it.  If necessary detach the wire connecting it to the unit.

*Note: The sensor may not be removable in some models, however you should still be able to clean it while it is in place.

3. Cleaning the Sensor

Using an emery cloth, fine grain sandpaper, or steel wool, carefully polish the metal rod until you have removed the buildup and any stuck on debris.  Give the flame sensor a final once over with a clean cloth or paper towel and inspect it for cracks or other signs of damage.

4. Replacing the Sensor

If the flame sensor is free of any signs of damage, simply reconnect the wire (if you disconnected it) and re-mount it with the screw, making sure it is positioned properly.  model_tag

If you did notice signs of damage, you may need to replace the sensor.  Flame sensors are not interchangeable, so it is very important that you get the correct part for your specific model (need help finding your model number?).  You can find the flame sensor you need for your furnace on our website.

5. Reconnect the Power and Test

Reconnect the power to your unit and turn the gas back on.  It may take a few extra minutes for your unit to start up, but from there it should be back to normal!

Did you find this post helpful?  Be sure to like and share it with a friend! Feel free to leave any questions, comments, or additional tips in the comments section below.  If you’re looking for more furnace tips, check out the Furnace Help section of our blog.

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