How To Test A Flame Sensor (With A Multimeter)

The flame on your furnace won’t stay lit. Instead, it comes on for a few seconds then goes off. So, you’ve diagnosed the furnace to point out the problem. Unsurprisingly, all signs point to a bad flame sensor. 

You’ve even called a more knowledgeable friend, and they also say you likely have a bad flame sensor. They’re even saying you should consider replacing it.

We also believe a replacement sensor may be necessary, especially if you’ve not done so in more than two years. However, before you head to the store to get a new sensor, it may be wise to test the sensor to verify if it is indeed damaged. 

The reason is that sometimes furnace flame sensors malfunction because of dirt buildup. A sensor can be so dirty that it loses its flame sensitivity. It cannot “see” or “feel” a flame when this happens, even if one is present. 

Fortunately, testing a flame sensor isn’t too difficult as long as you have a multimeter. Read on to find out the step-by-step process of testing a furnace flame sensor. 

How Does the Flame Sensor Work?

The flame sensor is a safety device that helps protect you and your loved ones from gas poisoning and potential fires. It also protects the furnace from potential flame rollouts that may damage the combustion chamber. 

It works by enabling/allowing gas supply to the burner only if there’s a flame in the burner. Otherwise, it disables the supply. 

To determine the absence of a flame in the burner, the sensor extends into the burner area and continuously feels out the area for heat. It comprises two main parts, a porcelain base that attaches to the burner assembly and a probe wire that dips into the area where the flame from the burner usually goes. 

When you ignite the burner, the wire detects the heat and sends the feedback to the furnace control board. The board then uses the information to enable or disable fuel supply. If a flame is present, it enables gas. However, if none is detected, it disables gas. 

Where is the Flame sensor Located?

The furnace flame sensor is located on the burner assembly inside the burner area. Some people even consider it part of the burner system. 

You can easily access it by opening the access door and checking inside the burner assembly. You’ll know you’ve found the flame sensor if you locate a thin metal strip with a white porcelain base. 

Remember that the white porcelain base can become discolored after a period of use. Often, it changes to a dingy brown or yellowish color. 

The thin wirehead can also take different shapes and colors depending on various factors. For instance, although it’s straight in many furnaces, some furnaces have bent flame sensors. The wire may bend at 45 degrees or 90 degrees. It may also be covered in soot. 

How to Tell if Your Flame Sensor is Bad

Furnace flame sensors can remain functional and in excellent shape for a long time. Some can go even three years without any issues.

However, ultimately, your flame sensor will misbehave. Either it can become too dirty to sense the flame or get damaged. In both cases, the sensor may lose the ability to detect a flame, requiring that you clean or replace it. 

The following is a three-step process to tell if you have a bad (not merely dirty) furnace flame sensor;

Check for Cracks on the Porcelain Base

The first surest sign that you’re dealing with a bad furnace flame sensor is a cracked base. The porcelain base serves multiple purposes. However, its primary purpose is to provide insulation to the bimetallic strips that constitute the flame sensor. 

As a result, a cracked base is often a likely sign of short-circuiting, i.e., two electrical parts that should be apart are touching each other. 

Check if the Wire Probe is Corroded

Unfortunately, the wire probe on the flame sensor is extremely delicate. It easily loses its capacity to detect a flame. Therefore, if the wire becomes corroded, it may lose its ability to sense flames. 

If you’re wondering how the wire may become corroded, the answer lies in the acidic wastes from the combustion process. Gas combustion inside the furnace burner produces several acidic wastes, including sulfuric acid, that can quickly eat up the surface of the probe wire. 

Rule Out Dirt and Positioning Issues 

Finally, you can rule out dirt and positioning issues, so you’re confident that the flame sensor is bad. Begin by checking the position of the sensor. Does the thin wire probe dip into the flame area? Does the fire surround the tip when you ignite the burner? If not, adjust the position as appropriate. 

Otherwise, take out the sensor and clean it thoroughly using a ball of soft steel wool. Then put it back, making sure to position it correctly, and test the furnace to see if the problem resolves. If it doesn’t remain lit, you likely have a bad flame sensor. 

What Happens When a Flame Sensor Goes Bad?

A bad flame sensor can result in all kinds of frustrations. For one, your furnace may refuse to come on no matter how many times you try. So, eventually, you may spend the night (or longer) in the cold.

Additionally, a bad flame sensor can cause furnace lockout. “Lockout refers to a state in which the furnace is unresponsive to any input from the user. Lockouts typically occur when you repeatedly try to start the furnace with a bad or dirty flame sensor. 

In both cases, you may need to hire an HVAC professional to fix the problem.

How to Test a Flame Sensor with a Multimeter

If the three tests above point to a bad flame sensor, the final step before replacing the sensor is to test it. A simple test will tell you whether it works or not. There are two ways to test a flame sensor;

The first option is to test it with a multimeter. A multimeter is a simple device that measures electrical current, voltage, and resistance. 

What you need;

  • An electric multimeter 
  • Screwdriver

Step 1: Shut Off Power and Gas Supply

Never attempt to repair or maintain the gas furnace with power and gas ON. It puts you at a massive risk of electrocution and gas poisoning. So, switch off the power at the furnace switch or breaker and turn off power at the gas valve. The supply is off if the valve is perpendicular to the gas line. 

Step 2: Access the Burner Area

To access the burner area, remove the access door. Depending on the furnace type, you may need to unlock a twist-lock or unscrew a few screws. However, some access doors also come up without unlocking or unscrewing anything. Just lift it and take it out. 

Step 3: Locate the Flame Sensor 

The furnace flame sensor is a thin piece of wire with a white ceramic base. If you can see a thin piece of wire that ends in the flame area, that’s likely the sensor. If it has a white porcelain base, you’ve found the sensor.

Step 4: Set the Multimeter to Ohms

Electric multimeters can measure many metrics. For this case, you need to measure resistance (in ohms). So, set the meter to ohms. 

Step 5: Test the Resistance

This is a two-step process. First, you need to measure the resistance of the sensor without heating. Then you also need to find out the resistance when it’s headed. 

Touch the multimeter probes on the sensor’s blue and white wire ports to check for resistance without heating. You should get a very low resistance, usually below 1.0 amps. 

To check the resistance when the sensor is heated, press the sensor’s tip (wire probe at the tip) against a 60-watt bulb or a test lamp. The bulb/lamp should be ON.

If the sensor is functional, it will give you a higher reading between 2.0 and 10.0 ohms. However, if there’s no difference in readings (with and without heating), you have a defective flame sensor that must be replaced. 

Step 6: Put Everything Back

Return the sensor, functional or damaged, where you found it, put back the access door, and restore power and gas supply by flipping back the switches as appropriate. Then think about your next move. If you verify that the sensor is damaged, plan how to replace it.  

How to Check Continuity of Flame Sensor

If you’re troubleshooting the flame sensor on a boiler, you can check the sensor’s flame continuity using the rectification flame. The rectification flame is present to help you prove that the boiler has fired. But it, by doing so, it will also tell you whether the flame sensor works.

It’s a simple process nonetheless. You need to apply a small AC to the flame rod. This way, when the rod is inserted into a proper flame, the voltage is converted to DC as it travels through the flame to the ground.

Putting a meter inline between the sensor and lead wire from the furnace control board allows you to read the DC. If the reading is below 5.0 microamps, you have a bad flame sensor. 


Testing the flame sensor is an important step in determining the functionality of the rod. If it works, you may only need to clean it and position it currently within the burner assembly. However, if it’s bad, you can plan for the replacement.