A flame sensor is a critical furnace safety component that verifies the presence of a burner flame. If the flame sensor cannot detect a proper flame signal, the furnace will shut off the gas supply to prevent unsafe operation. This guide explains common flame sensor problems, how to identify the symptoms, and what steps homeowners can safely perform before contacting an HVAC technician.
What Does a Flame Sensor Do?
The flame sensor is a small metal rod positioned in front of one of the burners. During a heating cycle, it detects a small electrical current carried through the flame. If the control board does not receive the expected flame signal within a few seconds, it closes the gas valve and stops the ignition sequence.
Common Symptoms of a Dirty or Failing Flame Sensor
- Furnace starts but shuts off after a few seconds
- Burners ignite briefly and then turn off
- Repeated ignition attempts
- Furnace enters lockout mode
- Intermittent heating
- Error codes related to flame sensing or ignition
Common Causes
- Dirty flame sensor
- Oxidation on the sensor rod
- Loose electrical connection
- Cracked porcelain insulator
- Poor furnace grounding
- Failed control board (less common)
When to Call an HVAC Technician
If cleaning the flame sensor does not resolve the problem, or if the furnace repeatedly enters ignition lockout, professional diagnosis is recommended. Additional testing may be required to verify flame-sensing current, grounding, ignition performance, gas supply, or control board operation.
Related Error Codes
Several Trane furnace error codes involve ignition or flame-sensing issues. As additional troubleshooting guides are published, this section will expand to include related diagnostic resources.