Low flame signal, furnace runs normally

What is going on

The furnace is still lighting and heating, but the flame sense micro amp signal is lower than it should be. A low signal can drop out and cause nuisance lockouts later, so catch it now. Shut the power off and let the burner area cool before reaching in.

Likely causes

• Flame sensor coated or oxidized
• Flame sensor not positioned correctly in the burner flame
• Lazy burner flame from improper gas pressure or combustion air

Check in this order

1. Turn power off before any repair and let things cool.
2. Clean the flame sensor with steel wool if it is coated or oxidized.
3. Inspect the sensor for proper alignment in the burner flame.
4. Check inlet air piping for blockage, proper length, elbows and termination.
5. Compare the current gas pressure to the rating plate and adjust as needed. Gas pressure adjustment is for a licensed gas technician.
6. Restore power, run a cycle and recheck the flame signal.

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)