How to Replace a Furnace Flame Sensor

furnace flame sensor
  • 3-4 hours
  • Intermediate
  • 400-600
What You'll Need
Replacement furnace flame sensor

If you have a furnace flame sensor in your heating furnace, you may need to replace it from time to time. Sometimes these sensors stop working properly after years of use, causing your furnace to cut out every few minutes. It can be very annoying.

If you have determined that your flame sensor is causing your furnace to malfunction, then removing it and installing another is the solution to this problem. Replacing the flame sensor on a furnace is not a difficult task. With some basic home-improvement, skills you should be able to complete this task within an hour or two. Follow these steps to replace your furnace flame sensor.

Step 1 - Locate the Furnace Flame Sensor

First find the flame sensor inside your furnace. Make sure that all heating is turned off and cannot travel into the furnace. Open the top using your screwdriver. You should be able to see a small L-shaped device which is hanging from the pilot burner, and with connections to the gas or electric circuits. This L-shape is the part that you need remove. You may have to remove the whole pilot burner set in order to access the flame sensor.

Step 2 - Get a Replacement

Once you have found your flame sensor and examined it, purchase a replacement. Read the owner's manual to obtain the sensor’s part number. Unlike ignition mechanisms, the flame sensor will need to have an exact part. Universal alternatives are not suitable for this device. Remember that it controls turning the heat off when the furnace reaches the correct temperature, so any malfunction in the sensor could cause the furnace to overheat.

Step 3 - Remove the Old Sensor

Once the furnace has been turned off, wait at least half an hour before touching the flame sensor. That is because it gets hot, and will be extremely dangerous to touch before this cooling period. The sensor may be permanently attached to the ignition system, in which case you will have to remove the lot. It may also be attached via a screw. You should remove the wire leads connected to the sensor, then remove the screws using your screwdriver or a wrench.

Step 4 - Add the New Sensor

Place the new sensor in position by the pilot ignition system. Your sensor should be facing the correct way, and have a clear view of the pilot light. When it is in the right position, tighten the screw.

You can then add the connecting electrical leads to the sensor, and attach the leads back to the electrical control box. If you had to remove the ignition, replace it now. You should then screw the access cover back down, and turn the electricity back on.