I know lit CELs must be cleared, but if the light is not on but the ECU is indicating a pending code, and a repair is made of the part that was causing the pending CEL (like O2 replacement), does this pending code have to be cleared or will the ECU clear it itself using new data from sensor?

 

I would just clear it myself.

 

ecu will clear itself for most if not all codes once the repair has been made.