Does the car charge when the EVSE is plugged in?
And the car fails to start because the charging plug light on the dashboard is flashing?
What is the history of the car, e.g. any accidents, repairs done, etc? where someone may have messed with wiring harnesses.
What is the age, condition and voltage of the 12V starter battery--a old, weak or worn out aux battery can cause numerous and sometimes false faults.
What DTCs where stored when the dealer determined the OBC was at fault before it was replaced? what diagnostic and troubleshooting steps were completed? what DTCs are now shown with the new OBC?
i would want to check the wiring harness from the front charging panel back to the OBC to ensure continuity and no damaged wires with two leads shorted together or to ground. Also inspect the connectors for debris, oxidation, damage, etc.