I believe that preventative maintenance for a flame rod involves cleaning it so anything on the surface does not insulate it causing a low microamp signal which would cut off power to the burners and a no heat situation if the flame is not sensed whether it is there or not.
I am not terribly familiar with thermocouples and the benefits and drawbacks of their usage for flame proving.
I believe that preventative maintenance for a thermocouple would involve checking its location in the flame and abilty to prove the flame through millivolts.
It seems as though the flame rod requires more consistent maintenance to work properly.
Why are thermocouples no longer used in furnaces?
I appreciate any insight.