Since a bad flame sensor is one of the most common furnace issues, it helps to know the location of the flame sensor on the furnace. But, more importantly, you should at least know basic flame sensor diagnostics to troubleshoot arising issues.
Below, we discuss the basics of the furnace flame sensor, so you know where to start next time you run into an issue.
What’s a Furnace Flame Sensor?
The furnace flame sensor is a short length of thin metallic rod found inside the burner assembly. It detects and confirms the presence of a flame burning inside the furnace to keep the gas supply alive or cut off supply as necessary to prevent poisoning and potential gas fire.
What Does the Flame Sensor Do on the Furnace?
The flame sensor is a safety feature. During furnace ignition, your gas furnace undergoes a process where either a spark or hot surface ignitor ignites the gas to generate the flame that ultimately provides warmth throughout your home.
The flame sensor’s role is to detect whether the ignition process is successful, i.e., whether it produces a healthy flame to facilitate heating.
The way it works is a little complicated. The sensor rod sticks out into the flame (or the area where the flame goes when you light the furnace) and connects back to the furnace control board. As such, the control board can constantly monitor current flow through the rod. The rod has a small voltage potential at the flame-sensing terminal.
When there’s no fire, there will be potential at the rod but no current. However, when there’s a fire, the ions from the flame interact with the voltage potential at the rod’s tip, creating a small direct current (DC) in microamps.
As we mentioned earlier, furnace control boards are extremely sensitive about current flowing through the flame sensor. As soon as the board picks up current within the rod, it gets the rest of the furnace into gear to commence heating.
The heating goes on smoothly as long as the flames sensor can sense a flame. However, if the flame goes out or the flame sensor cannot detect it for other reasons, it will shut the furnace down. Typically, the sensor waits 10 seconds before shutting down the furnace.
Modern furnaces allow you to try re-igniting the furnace twice. If the problem persists (the flame sensor cannot sensor a flame), the control board will lock out the system.
Where is the Flame Sensor on the Furnace?
The flame sensor is easy to find. It’s located within the burner behind the furnace access cover. Usually, it’s held in place by tabs and slots, knobs, and a few screws.
To access it, perhaps to check it after a malfunction, proceed as follows;
- Disconnect the furnace: Never attempt to access the burner area with the furnace running. It exposes to the risk of burns and electrocution. So, begin by shutting off the power at the switch. Then close the gas valve.
- Allow the furnace to cool: The internal components will be very hot if the furnace runs for more than 10 minutes. So, let it cool down for at least 15 minutes before you begin the diagnosis.
- Remove the burner compartment door: This process varies from one furnace model to the next. However, most furnaces have twist locks on the door and screws securing the panel. Meanwhile, others don’t have anything securing the door. So, you need to lift it from the bottom outward to access the burner area.
- Locate the flame sensor: The flame sensor is a metal rod with a white or dingy yellow porcelain surrounding the base. It’s mounted outside the burner assembly, with a single wire attached to it.
Most sensors are straight. However, others have a 45-degree or 90-degree bed near the end. You can also tell you’ve found the flame sensor when you locate a thin wire that protrudes into the open fire chamber or through the housing into the interior where the flame typically burns.
How to Tell if Your Flame Sensor is Bad
It’s never easy to know for sure that your flame sensor is bad until you physically test it. Nevertheless, you should keep an eye out for three signs;
- The furnace lights up then shuts down after a few seconds: The flame will go off after a few seconds if the flame sensor doesn’t detect the fire and inform the control board of the same. The porcelain on the flame sensor is cracked: A cracked flame sensor is an obvious sign of wear. It tells you that perhaps it’s time to replace the sensor.
- The flame sensor is sooty and corroded: A sooty and corroded flame sensor is also a sign of wear. However, it’s an even bigger sign of exposure to external elements. A dirty, sooty, and corroded flame sensor may lose its flame sensitivity. If you see a yellow flame in furnace you also need to fix it immediately
Furnace Flame Sensor Testing
To verify that you indeed have a bad flame sensor, you need to test it. Fortunately, it’s a simple process that doesn’t require professional/expert input.
To get started, locate the flame sensor by following the steps in the “where is the flame sensor on the furnace” section. Once you locate the sensor, proceed as follows;
- Pull out the sensor: Use a screwdriver or wrench to unfasten the screws holding the flame sensor in place. Then carefully pull it out.
- Measure its resistance: You need a multimeter for this step. Touch the multimeter probes to the white and blue wire ports on the flame sensor. It should detect a low resistance.
- Warm the sensor and test the resistance again: Hold the flame-sensing end against a 60-watt bulb or higher or dip it into a candle flame for about five seconds. Then recheck the resistance. This time the resistance should be high.
- No change in resistance? If there’s no change in resistance when the flame sensor is cold vs. when it is warm, the sensor has malfunctioned and needs to be replaced.
How to Bypass the Flame Sensor on the Furnace
Unfortunately, you cannot bypass the flame sensor and light your furnace manually. The gas valve is designed only to open when it receives such instruction from the control board. Meanwhile, the control board can only request the gas valve to open if the flame sensor detects a flame.
How Much Does it Cost to Replace a Furnace Flame Sensor?
It depends on many factors, including where you buy it, the type of flame sensor (brand, durability, etc.), and whether you DIY replace it or hire an HVAC professional.
Generally, the flame sensor costs $6 to $75, depending on the furnace model and whether it’s an OEM part of a universal sensor. Most units cost under $40, though. So if you decide to install it yourself (DIY), that’s all you have to worry about.
However, if you opt for professional service, which we strongly recommend, you’ll pay for the cost of the sensor plus a small service charge. The minimum service charge ranges from $65 to $150, depending on location. When you combine this with the cost of the sensor ($6-$75), you find that you’ll pay between $71 and $225 for the entire job.
The flame sensor is a critical component of the gas furnace that keeps you and your home safe from gas poisoning and potential fires. Therefore, you must make sure it’s working correctly at all times. If you can’t locate the problem, call an HVAC professional.
Also make sure you clean furnace flame sensor as per your manufacturer guidelines.