What the Flame Sensor Does
The flame sensor detects burner flame presence by measuring electrical current through the flame. If no flame is detected, the gas valve closes for safety.
Symptoms of a Dirty Flame Sensor
- Furnace ignites then shuts off after a few seconds
- Repeated ignition attempts
- Lockout after 3 failed attempts
- Error code for flame failure
Cleaning Procedure
- Turn off power and gas to furnace
- Locate flame sensor (thin metal rod near burners)
- Remove mounting screw
- Clean with fine steel wool or emery cloth
- Reinstall and test operation
When to Replace
Replace if cleaning doesn't resolve the issue, or if the sensor shows cracks or damage. Most flame sensors cost under twenty dollars.