Fair enough. I'm speaking broadly in order to reduce the failure paths with an aircraft and mechanic that I am not familiar with, over the internet. The issue is that the ECU needs to be powered up. There are scenarios where depending on the versions of the hardware and software and the sequence and timing of this mechanic's process, the fuse box power could be timing out. If that happens, the dongle will not be able to read the ECU. If the EMS switch is set to power the fuse bot, this failure path is eliminated.
Anyway, it's probably just the pins reversed.
