A Complete Guide to Furnace Flame Sensors

A flame sensor is one of the most important safety features of your furnace. This component is responsible for ensuring that your furnace doesn’t create a dangerous gas leak or explosion.

By understanding why the furnace flame sensor is crucial to the unit’s operation, you’ll be better prepared to recognize when a flame sensor is bad. Our guide will walk you through cleaning the sensor and furnace flame sensor replacement.

What is a flame sensor?

The flame sensor is a safety component of a furnace located on the interior of the burner assembly. It monitors and verifies whether a flame is burning inside the unit.

If the sensor does not detect an active flame, then it will shut the furnace off to avoid a potential gas leak. The furnace flame sensor helps prevent explosions that could occur if gas were allowed to continue to flow into the home as well as carbon monoxide poisoning of those inside the home.

The Centers for Disease Control and Prevention report that at least 430 people die in the U.S. from accidental carbon monoxide poisoning each year and some 50,000 visit the emergency room because of the gas. While home furnaces are responsible for only a small percentage of these cases, it’s a worthwhile reminder to install a carbon monoxide detector on each level of your home.

Flame sensor vs. thermocouple

A flame sensor and thermocouple are often referred to as the same or interchangeable parts, but that’s not the case. While they both serve as a safety feature on the furnace, you would need to know which you have in order to replace or repair the component.

A thermocouple is often found on older model gas furnaces with a standing pilot. It has a small flame that burns continuously, keeping the top of the thermocouple at a high temperature. If the flame goes out and the tip of the thermocouple cools, it will automatically shut off the furnace’s gas valve.

Flame sensors are used in modern furnaces that operate via an electronic ignition rather than a standing pilot light. These models use electronic igniters to light the gas. As mentioned above, the flame sensors ensure the burners are operating properly and have ignited the gas. If the sensor detects that the burner has failed, it will turn off the gas.

Furnace flame sensor location

When trying to determine how to find a flame sensor, you’ll want to remove the furnace access cover to begin. Depending on the model of your furnace, you may have to remove screws or bolts to release the access cover.

The sensor is located just outside the burner assembly and looks like a small rod with porcelain surrounding the end that connects to the burner. Some furnace flame sensors are straight while others bend at a 45-degree or 90-degree angle. The sensor leads into the fire chamber where the flame burns.

Signs of a bad flame sensor

As with any other component in your heating and cooling system, the flame sensor can go bad. If your furnace stops working properly, look for warning signs that your flame sensor has failed:

• Furnace burners turn on but go out after a few seconds
• Soot and other debris is covering the end of the sensor (cleaning the flame sensor may fix this issue rather than a flame sensor replacement)
• Tip of flame sensor is black
• Porcelain/sensor casing is cracked

If a flame sensor replacement is necessary, it’s a low-cost fix that you can likely handle yourself if you feel comfortable working with the unit. Most furnace flame sensors cost less than $20, but if you opt to have a professional HVAC technician make the repair, you can expect to pay between $75 and $250.

Flame sensor lifespan

If you’ve just replaced the sensor or had a new furnace installed, you can expect the flame sensor to last for about five years. The average lifespan of a furnace is 15 to 30 years, so you can anticipate a flame sensor replacement once or twice over the course of your homeownership.

To extend the lifespan of the sensor, seasonal furnace maintenance and cleaning the flame sensor is necessary. Even if the furnace seems to be operating smoothly, cleaning the sensor each year can prevent you from calling for emergency heating services in the middle of winter.

Furnace flame sensor cleaning

Before you begin the flame sensor cleaning process, turn the gas and electricity off to the unit. The valve handle on the gas pipe will allow you to stop the gas flow and turning off the furnace breaker at your electrical box will ensure all power is shut down.

If the furnace has been running, wait at least 30 minutes before attempting these steps:

• Locate the flame sensor (refer to the furnace flame sensor location portion above)
• Remove the furnace access cover, which may involve releasing screws or bolts
• Unfasten/unscrew the flame sensor from the burner assembly
• Disconnect the wiring that leads to the sensor and control box
• Loosen the screws and remove the flame sensor
• Use an emery cloth to gently remove soot and other debris from the sensor
• Reattach the sensor
• Reconnect the wiring to the sensor and control box
• Secure the flame sensor to the burner assembly
• Reattach the furnace cover access

Test the furnace to see if cleaning the flame sensor was the solution you needed. If the unit turns on and off properly, a flame sensor replacement may not be necessary.

Replace furnace flame sensor

During the sensor cleaning process, if you discover the tip of the sensor is black or the component is misshapen or broken in any way, replace rather than repair the part. Follow the flame sensor cleaning steps, but install the new sensor instead of cleaning the old one.

Your furnace will only operate with a certain type of flame sensor. Ensure the component you purchase is compatible with your unit. Check the owner’s manual for guidance on the type of sensor you need.

If working with a furnace or handling the gas line is not within your DIY comfort level, take the safe route and call an HVAC expert. A furnace specialist will test the flame sensor and replace it if needed.

To avoid being caught in a situation where furnace service is needed immediately, schedule routine inspections. Seasonal maintenance can support your unit in working properly when you need it most and expanding the lifespan of the furnace. Explore our top furnace brands if it’s time to replace your unit.