Flame sensor cleaning and signal check

What is going on

Dirty flame sensor. Fuel or air supply leaves a nearly invisible film on the rod. It insulates the rod and drops the flame signal. Qualified servicer cleans it with steel wool, gently. Then check the sensor output against the spec sheet.

Likely causes

• Insulating film on the flame sensor from fuel or combustion air cuts the signal
• Sensor rod bent or out of the flame path after handling (check when reinstalling)

Check in this order

1. Power off at the disconnect. Let the burner area cool.
2. Remove the flame sensor carefully, don't bend the rod.
3. Clean the rod with steel wool until bright. No emery cloth that leaves grit.
4. Reinstall the sensor in the same position, tighten the mounting screw.
5. Power up, run a cycle, check flame sensor output against the spec sheet.
6. Signal still low after cleaning? Look at burner flame and sensor wiring.

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)