If your gas furnace keeps cycling on and then turning off within a minute or so, it might be the result of a dirty flame sensor. Most gas furnaces have a small sensor designed to shut off the furnace if the burners do not flame when the furnace is turned on. Over time the sensor can become coated with carbon deposits and dust, causing the furnace to turn off even when the burners are lit. Getting the furnace up and running again may be as easy as cleaning the sensor.
Step 1: With the control switch on the thermostat in the off position, shut off the electrical power to the furnace. This can be usually be done using a switch located on or near the furnace. If not, use the circuit breaker or fuse in the electrical panel.
Step 2: Remove the service panel on the furnace. It is typically held in place with several screws.
Step 3: Locate the flame sensor. The sensor will be somewhere near the burner with the thin metal rod of the sensor pointed toward the burner flame. A single electrical wire will be attached to the porcelain base of the sensor with an insulated wire connector. The sensor should be attached to the furnace with a metal mounting bracket and a single screw. On some furnaces, it may be necessary to remove the burner plate to access the flame sensor.
Step 4: Disconnect the wire attached to the sensor by pulling on the insulated wire connector.
Step 5: Unscrew the flame sensor mounting bracket screw and gently pull the sensor out from inside the furnace.
Step 6: Check the porcelain base of the sensor for cracks or chips. If the porcelain is compromised in any way the sensor should be replaced.
Step 7: Use fine or extra fine steel wool to clean the carbon and dirt off of the sensor’s metal rod. Do not use sand paper to clean the rod. The grit from the paper can melt into glass on the rod and inhibit the rod’s sensing abilities.
Step 8: Once the rod is shiny and clean, re-install the sensor and attach the electrical wire.
Step 9: Replace the cover panel and test the furnace to see if cleaning the flame sensor solved the problem.
Step 10: If the furnace is still not functioning properly, it is probably time to call a professional.
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