With the weather turning chillier by the day, now is the time to make sure your furnace is in prime operating condition before your comfort really depends on that steady stream of heated air coming through your home’s venting. In addition to changing the air filter (which we recommend doing at least twice a year), you should turn on the furnace and monitor the system’s thermostat to ensure it’s working properly. If the furnace starts then stops unexpectedly, or the heat doesn’t seem to come on at all, a dirty or defective flame sensor could be to blame.
What does a furnace flame sensor actually do?
The flame sensor monitors the burner to detect whether or not a flame is present. If the furnace’s igniter is unable to ignite the incoming gas to produce the flame that will heat the air, a properly working flame sensor will detect the absence of the flame and alert the control board to shut off voltage to the gas valve. This will close the valve and prevent excess gas from seeping out of the furnace and into the surrounding area. Consequently, the furnace will appear to start then stop or not heat the air being forced through the vents.
After years of use, the flame sensor can collect carbon build-up that will interfere in its ability to detect a flame. If this happens, the control board will continue to shut off voltage to the gas valve even though the igniter, gas valve, and burner assembly are working fine. Cleaning the flame sensor can restore its ability to properly detect a burner flame and allow furnace operation to continue unimpeded.
Accessing the flame sensor
The flame sensor is usually mounted on or near the burner assembly in the furnace. While the steps required to access the sensor can vary depending on the furnace model, there are some general guidelines you can follow:
- For safety, be sure to shut off the power to the furnace before you remove any panels. You should shut off the gas supply as well.
- Depending on the model, you may be able to simply slide the upper or lower compartment access panel upwards to detach. Otherwise, you will probably need to rotate or unthread two retaining knobs, or unthread mounting screws, before you can remove the panel.
- The flame sensor is usually secured to the burning housing with a single mounting screw. Unthread the screw and carefully pull the sensor out of the housing.
- Keep in mind, you may need to first disconnect the wire from the sensor to fully remove it from the housing or to have better access to the sensor to clean it.
Cleaning the flame sensor
You can use Emery cloth sandpaper or a fine abrasive pad to gently clean any build-up from the flame sensor’s sensing rod. If present, you should also clean off any corrosion where the power wire connects to the flame sensor terminal. Use a lint-free cloth to wipe the component clean of any remaining debris.
- When you’ve finished cleaning the sensor, reseat it in the burner housing and secure it with the mounting screw.
- Reconnect the wire to the sensor terminal.
- Reposition the access panel and secure it with the mounting screws or retaining knobs as required.
- Turn the gas supply back on and restore power to the furnace.
- Call for a higher temperature on your home’s thermostat and observe if the furnace now appears to be functioning normally.
Of course, the flame sensor could simply be defective and unable to work at all. While a defective sensor is the most likely reason a furnace would start and then stop, keep in mind that other components, such as the igniter, the pressure switch, and the control board, could be faulty as well.
Find the right furnace parts with Repair Clinic
If the flame sensor is definitely the culprit, you can purchase a new one from Repair Clinic. To find the right sensor that fits your model, type the full model number of your furnace into the Repair Clinic website search bar, then select “Sensor & Thermistor” using the part category filter followed by “Flame Sensor” using the part title filter. Repair Clinic stocks flame sensors, and other furnace replacement parts, for all the leading heating and cooling brands, including Bryant, Lennox, Goodman, Rheem, York, Coleman, Ruud, Heil, and Payne, but it’s important that you identify the specific sensor that matches your model for the part to work properly.