Furnace Flame Sensors: How They Work, When to Repair or Replace

When your gas furnace shuts down repeatedly or shortly after turning it on, you may have a furnace flame sensor issue. Although many homeowners experience furnace flame sensor problems, they rarely need to make expensive heating repairs. You can identify a faulty furnace flame sensor when you understand why it is essential to its proper operation. Keeping a furnace flame sensor clean and working efficiently can prevent more furnace flame sensor issues and the need for professional HVAC service.

What is a Furnace Flame Sensor

Furnace flame sensors detect the flame to confirm the furnace has ignited. They will shut off gas flow if it detects no flame at a time when the furnace is supposed to be running.

A defective flame sensor, however, won’t detect the heat, and the furnace will shut off after it has been ignited and is burning hot. In the case of open gas valves, but the furnace is not turned on, it isn’t just releasing raw natural gas or propane. It can easily explode if a water heater or gas stove is turned on.

How to Find It

Flame sensors are metal rods with porcelain ends encased in white or yellow/brown porcelain. The porcelain protects the sensor from grounding on the furnace frame. Some sensors feature a straight end, while others have a 45- or 90-degree curve. They are under the furnace access cover, and it’s usually held in place with tabs and slots, knobs, or screws.

A single wire connects the flame sensor to the burner assembly from the outside. A flame sensor extends into a fire chamber or into the interior of the enclosure where a flame is burning.

How to Diagnose a Flame Sensor on Your Own

You can use these tips to determine whether your flame sensor is malfunctioning or simply dirty:

  • Turn off the electricity and shut off the gas to your furnace.
  • Remove the mounting screw, and be careful when removing the sensor.
  • Check the sensor: if the insulation is intact but appears sooty, burnt, or corroded, cleaning is necessary.
  • If the insulation on the sensor is badly damaged or missing, a replacement is necessary.

What You Need in Order to Clean it

Cleaning a furnace flame sensor is easy, provided you have the right tools. Here are five tools you’ll need:

  • A wrench or a driver, depending on the type of screw that is holding your furnace flame sensor in place
  • Steel wool, fine-grit emery cloth, or a wire brush
  • Paper towel or a clean, dry cloth

You can clean your flame sensor by following these steps:

  • Turn off the furnace. Look for the metal rod-shaped sensor in the furnace.
  • Remove the screw with a driver or wrench so you can see the sensor.
  • You should gently rub the rod with steel wool, emery cloth, or wire brush to remove the carbon buildup, taking care not to overdo it.
  • Wipe the sensor with a cloth or paper towel as soon as you are done cleaning.

How to Change a Flame Sensor

When you remove the sensor and find that it is clearly broken or that cleaning it doesn’t help, it needs to be replaced. Look for the brand, serial number and model number inside the access panel of your furnace. You should be able to find the sensor at a local furnace parts supplier or an online furnace part supplier.

An exact replacement item may not always be necessary. Home Depot, Lowe’s, and other building supply stores sell similar universal sensors. Buying a sensor will cost between $6 and $75, depending on whether you choose a universal sensor or a brand-specific sensor.

The steps below can help you replace a flame sensor.

  • Take the mounting screw out and remove the sensor.
  • Disconnect the sensor’s wire
  • Place the new sensor in the gap.
  • Use the mounting screw to keep it in place.
  • Add the new sensor to the wire.

What to Do if Your Furnace Still Doesn’t Work?

In most cases, you can fix the problem by cleaning or replacing the sensor if your furnace starts and shuts down within a few seconds. When the replacement sensor does not work, your best bet is to contact a qualified HVAC repair company to figure out why your furnace is malfunctioning.

Taking Care of Your Furnace Sensor

Clean the furnace’s flame sensor if necessary, and replace it if it’s damaged, corroded, or dark. Having regular maintenance will help you avoid having to call for emergency furnace repairs. Maintaining and caring for your sensor and furnace will keep you and your family safe no matter the season.