Gas Furnace Flame Sensor ProblemGas Furnace Number 1 Problem Flame Sensor - What is the number one trouble call for gas furnaces? We went through our records and found the number one problem for a gas furnace troubleshooting call is the flame sensor. If the flame sensor is not functioning as designed it will prevent the gas furnace from firing. The behavior of the gas furnace when the flame sensor fails will be trial for ignition and eventually a lockout. This is a common problem. So, how do you repair this problem and get your heat working again?

Gas Furnace Number 1 Problem Flame Sensor

How to Recognize the Problem

If you are not comfortable working on a gas furnace or have no practical skills or tools call for service. For this job, you will likely need a quarter-inch nut driver and a fine-grain piece of sandpaper. Again, if you are not comfortable with this then call for service. This procedure is for an electronic ignition gas furnace and not a standing pilot gas furnace.

On a call for heat from the thermostat, the furnace will go through its sequence of operations. The induced draft motor will energize. Shortly thereafter, the burners will ignite and then quickly extinguish. In some furnaces, this burner short cycling will occur three or four times and then the furnace will lock out for a period of time. The lockout will typically be for 4 hours or more. To reset the lockout timer simply reset the power to the furnace.

The problem will occur again and then the furnace will lock out. If your gas furnace is exhibiting this behavior then you likely have a problem with a dirty flame sensor and it needs to be cleaned. The following procedure will help you repair the gas furnace.


Turn Off the Power

gas furnace power switch

To fix this problem of the flame sensor not allowing the furnace to light we must clean the flame sensor. In some furnaces, this is easily done while with other gas furnaces the flame sensor is difficult to remove for cleaning.  Before searching for the flame sensor for removal and cleaning we recommend you turn off the power to the furnace. For extra caution we also recommend you turn the power off at the breaker for the gas furnace in the main breaker panel. This is a basic safety requirement to prevent the gas furnace from trying to fire when you are working on it.


Find the Flame Sensor | Gas Furnace Number 1 Problem Flame Sensor

flame sensor burner compartment

Inside the burner compartment, the flame sensor will be located in front of one of the in-shot burners. Be careful not to mistake the igniter for the flame sensor. There are different types of ignition systems in an electronic ignition gas furnace. If you have a hot surface ignition system, the hot surface ignitor can be fragile and easily break. A flame sensor is hard and made of hard metal while the hot surface ignitor is made of silicon carbide or silicon nitride. In this case, the furnace is lighting but not staying lit so there is no problem with the igniter. The most common cause of this is a dirty flame sensor.

burner compartment gas furnace


Cleaning the Flame Sensor | Gas Furnace Number 1 Problem Flame Sensor

You can follow the wires in the burner compartment. One will go to the flame sensor and the other will go to the ignitor. In most cases, one is on one side of the burner compartment while the other is on the opposite side of the burner compartment. In some cases, the flame sensor is accessible while in other cases it will need to be removed for cleaning. If it is accessible use a fine grain of sandpaper and lightly sand from the tip to the base of the flame sensor.  It is important to lightly sand the flame sensor. It does not take much to clean it.

If it is not accessible you will have to remove the flame sensor. It is important to make note of the position of the flame sensor before removing it. You will have to reinstall the flame sensor in the same position as you removed it. You may take a photo of it before removing the flame sensor. The flame sensor needs to be in the flame and reinstalled the way you found it. Once it is removed, clean it with the sandpaper as noted above. Be careful not to use too much pressure and use only a light sanding of the sensor.

Conclusion | Gas Furnace Number 1 Problem Flame Sensor

Once you have cleaned the flame sensor make sure no wires from the igniter or flame can get in direct contact with the flame. Once the flame sensor has been cleaned and reinstalled turn the power back on. The gas furnace should fire and you will have heat once again. If this does not resolve the flame sensor problem then you will likely need to call for service. It could be another problem such as a bad circuit board. Good luck!!

High Performance HVAC Heating & cooling

Gas Furnace Number 1 Problem Flame Sensor