Symptoms
- Furnace ignites normally, then shuts off after a few seconds
- Furnace tries to restart, repeating the short cycle a few times
- Eventually locks out and needs a thermostat reset to try again
Likely causes
- CommonCarbon buildup coating the flame sensor rodA thin layer of soot insulates the sensor so it can't 'see' the flame's electrical signal.
- PossibleFlame sensor out of positionIf it's bent or not sitting in the flame path, it won't register properly even when clean.
- PossibleWeak flame from dirty burnersA weak or discolored flame may not generate enough signal even with a clean sensor.
- Less commonFailed flame sensor or control boardAfter ruling out dirt and position, the sensor or board itself may need replacement.
What to do
- Turn off power to the furnace at the switch or breaker before touching anything inside.
- Locate the flame sensor, a thin metal rod near the burners, usually held by one screw.
- Remove the single mounting screw and gently pull the sensor out.
- Clean the rod with fine sandpaper or steel wool until it looks shiny, not gray or sooty.
- Reinstall the sensor exactly as it was, restore power, and run a full heating cycle.
- If it still shuts off after cleaning, the sensor position, burners, or the sensor itself may need professional attention.
Tools and parts
- Fine sandpaper or steel wool
- Screwdriver
Furnace flame sensorSearch: “universal furnace flame sensor rod replacement”
When to call a pro
Call an HVAC technician if cleaning the flame sensor doesn't stop the short cycling, or if you're not comfortable opening the furnace cabinet.
Safety
Use caution
Always turn off power at the switch or breaker before opening the furnace cabinet or touching the flame sensor.