Does the furnace gas turn off when you switch on the furnace? If so, there could be a problem with the furnace flame sensor. This is especially true if the furnace shuts down completely after several attempts. The furnace flame sensor is a thin, usually bent, metal rod located in front of the furnace flame stream. A flame sensor is designed to show homeowners the gas valve is open when there is fire. This is to help prevent gas from building up. Flame sensors are included in modern furnaces as well as other modern gas-fired heating systems.

How Do I Know if My Furnace Flame Sensor Is Bad?

Flame sensor problems are fairly generic and fortunately aren’t complex or expensive to fix. If you are having problems, it could just be the case of a dirty or faulty flame sensor. However, it is best to contact a Heating Ventilating, and Air Conditioning (HVAC) professional.

Worn-out Flame Sensors

It is common for flame sensors to wear out over a period of time. However, more often than not, a worn-out flame sensor is usually a sign it is dirty, rather than broken. These sensors have limited tolerance for reading variations by design. Therefore, they might miscalculate readings from even minimal carbon coating. Furnace locations, typically laundry rooms and basements, are dusty places. As a result, airborne particles tend to gravitate to and stick to the flame sensor and then burn, causing a buildup of carbon. Give the sensor a good clean and see what happens.

Bad Flame Sensor

If the furnace doesn’t work after the flame sensor has been cleaned, you might need to replace the furnace flame sensor. If the sensor doesn’t detect a flame, it should automatically close down the heating unit. Caution, if the flame sensor is worn out or broken, the furnace will keep emitting gas despite there being nothing to ignite. This can be extremely dangerous as the gas will build up. Therefore, if the furnace fails to ignite after three attempts it should default to a safety ignition lockout. Do not attempt to keep igniting the furnace.

Flame Sensor Inspection

Use the following guidelines in order to help you to determine if the flame sensor is simply dirty or has gone bad.

  • Turn off the power supply to the furnace. Double-check this.
  • Shut the gas valve
  • Remove the mounting screw
  • Gently take out the sensor
  • Examine the sensor
  • Should the insulation still be intact even though it may be a touch corroded or burnt, it will only need to be cleaned and the soot removed.
  • If the insulation is cracked or there are pieces of insulation missing, it will need to be replaced.

How to clean the furnace flame sensor

Many furnace flame sensor problems can be resolved simply by cleaning the sensor. This should be a part of the service provided by the professionals during a regular, routine furnace tune-up. However, if you are comfortable with cleaning the flame sensor yourself, do as follows:

  • Shut down the power to the furnace.
  • Then, remove the flame sensor.
  • Next, remove the carbon buildup on the sensor cautiously and diligently. This can be done by using sandpaper, steel wool, or a wire brush. Be careful not to over-clean the rod.
  • Once the rod is clean, wipe the rod with a cloth or some paper towel.
  • Reinstall the sensor, turn on the power and test the furnace.


Replacing a flame sensor is a job best left to trusted HVAC professionals. Therefore, hire an HVAC expert to examine the furnace to identify the exact problem. DIY furnace flame sensor replacement and tampering can lead to other problems and a reduction in the furnace’s efficiency. Avoid an expensive mistake, call the furnace experts!.

Additional Resource: Daikin User Manuals

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