furnace flame sensors in gas furnace
furnace flame sensors in gas furnace

If your heating system suddenly stops working correctly, the furnace flame sensor might be the cause. The flame sensor shuts the furnace down when it senses a malfunction involving the furnace’s gas valve. The flame sensor also shuts the furnace if the sensor is malfunctioning.

If it turns out that the furnace shut down due to a malfunctioning gas valve, the flame sensor did its job to prevent an explosion or carbon monoxide poisoning. Furnace issues of this type require professional expertise to resolve.

A more likely cause of your sudden furnace shutdown is a malfunctioning flame sensor. A dirty or faulty flame sensor will disrupt the furnace’s operation. Luckily, flame sensors are easy to clean and replace. Both processes involve a few simple steps–loosening a few screws, disconnecting some wires, scouring the part’s residue, or swapping the part out with a new one. Of course, you can call a professional to perform these tasks if you aren’t comfortable doing them yourself.

How Does a Gas Furnace Flame Sensor Work?

A gas furnace flame sensor is a safety feature. The flame sensor detects whether the burners are lit when the furnace's gas valve opens. If no flame is detected, the sensor will cause the furnace to shut down to prevent explosions from gas leaks and deadly carbon monoxide poisoning.

Carbon monoxide poisoning is a potential risk of all gas furnaces. According to the CDC, carbon monoxide poisoning affects tens of thousands of Americans annually, resulting in over 400 deaths and more than 4,000 hospitalizations yearly. Flame sensors, in addition to well-placed carbon monoxide detectors, reduce the risk.

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Flame Sensor vs. Thermocouple

Flame sensors and thermocouples are both furnace safety components that sense heat and prevent gas leaks. However, the two parts function differently and are not interchangeable. Thermocouples are used in furnaces with pilot lights; flame sensors are used in furnaces with electronic ignitions.

The flame sensor’s primary purpose is to detect whether the burners are lit. It cannot be used as a thermocouple. The flame sensor shuts off the gas valve if the burners aren't lit. It resembles a thin, usually bent, metal rod with a porcelain-coated mounting end.

The thermocouple’s primary function is to produce a current for other purposes, but it can also function as a flame sensor. The thermocouple sits in the pilot light’s flame and senses whether it’s working correctly. If the flame goes out, the thermocouple’s metal tip cools and shuts off the gas valve. A thermocouple is made of copper tubing, a bracket, and wires.

Flame sensors and thermocouples cause furnaces to malfunction when soot builds up on them, and they can easily be cleaned using abrasive materials.

Where Is the Furnace Flame Sensor?

The flame sensor is located on the burner assembly where the flames burn. To access it, shut off the power to the furnace and remove the access panel. The sensor resembles a thin metal rod with a porcelain-coated mounting end. The rod may be straight, bent 45 degrees, or bent 90 degrees.

gas furnace illustration showing flame sensor location

Signs of a Bad Gas Furnace Flame Sensor

Flame sensors malfunction if they are dirty or faulty. Four signs indicate a malfunctioning flame sensor:

  1. The furnace continuously turns on and off.
  2. The furnace makes a clicking sound.
  3. The flame sensor’s tip is black, dirty, or corroded.
  4. The porcelain casing is cracked.
woman adjusting thermostat because of furnace short cycling

The furnace continuously turns on and off.

A classic sign of a bad flame sensor is when the furnace continuously turns on and off or short cycles. The thermostat signals the furnace to turn on, the burners ignite for a few seconds, and the flames go out. The thermostat senses the temperature is below the setting and sends another signal to turn on the furnace. This progression repeatedly occurs because a dirty or faulty flame sensor cannot sense the burners’ flames and shuts off the gas after they ignite.

woman listening to noise from furnace  clicking

The furnace makes a clicking sound.

The clicking sound comes from the furnace’s ignitor repeatedly attempting to light the furnace. A bad flame sensor cannot detect the burners’ flames and shut off the gas. The ignitor continuously tries to relight the burners. The ignitor will eventually wear out if the issue is not resolved.

dirty furnace flame sensor

The flame sensor’s tip is black, dirty, or corroded.

Visible black soot, sediment, and corrosion on the tip of the flame sensor impede its ability to sense flames. The sensor can easily be cleaned to restore it to working order.

furnace flame sensor with cracked porcelain

The porcelain casing is cracked.

The flame sensor’s porcelain casing insulates and separates the bimetallic strips to maintain voltage. A cracked case can cause electrical components to touch each other, short-circuit, and damage the flame sensor.

How to Clean a Furnace Flame Sensor

Regular flame sensor maintenance is crucial to keeping your furnace running, and it’s easy enough for homeowners to do themselves. There are six steps to clean a furnace flame sensor:

  1. Shut off the power.
  2. Locate the flame sensor.
  3. Remove the sensor.
  4. Clean the sensor.
  5. Replace the sensor,
  6. Test the sensor.

Step 1: Shut off the Power

Shut off power to the furnace at the toggle switch on the furnace or the circuit breaker panel. Turning off the heat at the thermostat is inadequate and does not cut power to the unit. Shut off the gas valve if your furnace doesn’t use an electronic control. If the furnace has recently been running, let it cool for thirty minutes before working on it.

Step 2: Locate the Flame Sensor

The furnace’s manual can help you locate the flame sensor. Generally, the access panel needs to be removed with a screwdriver to get to the flame sensor. The flame sensor resembles a thin, possibly bent metal rod extending near the burners.

Step 3: Remove the Sensor

The sensor is usually mounted with a quarter-inch hex screw and attached to wires. Remove the screw, and carefully slide the sensor out of the housing. Disconnect the sensor from the wires.

Step 4: Clean the Sensor

Gently rub the metal section of the sensor with a wire brush, straight-edge knife, or abrasive material, such as fine-grit sandpaper, steel wool, or emery paper. Rub hard enough to remove the residue, but be careful not to damage the sensor. Do not rub the porcelain with anything abrasive. Wipe the sensor clean with a paper towel or cloth.

Step 5: Replace the Sensor

Reconnect the sensor’s wires, slide it back into the housing, replace the screw, and restore the access panel.

Step 6: Test the Sensor

Turn on the power to the furnace. The furnace might do a system reset and take longer to fire up than usual. Make sure the burners eventually ignite and stay lit for a complete heating cycle. If the furnace malfunctions, the flame sensor might be faulty. Replace the sensor or call an HVAC professional to diagnose the problem.

When Should You Replace the Flame Sensor?

Replace the flame sensor if it’s cracked or broken or if the furnace continues to malfunction after cleaning it. Flame sensors last about five years and must be replaced several times during a furnace's ten to twenty-year lifespan.

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A malfunctioning flame sensor is a common furnace issue. If your furnace short cycles, clicks continuously, or shows other signs of a bad flame sensor, clean the flame sensor. Oftentimes, your furnace will be up and running again after a quick cleaning.

If your furnace continues to malfunction or you discover the sensor is damaged, replace the flame sensor. Flame sensors only last about five years and don’t correctly function when damaged.

If cleaning and replacing the sensor won’t resolve your furnace issues, it’s time to call an HVAC professional. Avoid furnace issues in the future by scheduling yearly furnace maintenance inspections.


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