Have the trucks been modified? As in programmed with a programmer like sct, bullydog, hp, etc, for reasons such as egr delete, power, towing, it doesn't matter why, but to get to why I ask. If it has been programmed with a burner, then most of them lock the pcm so you can not access it. If this is the case you need the programmer that is married to the vehicle then return the vehicle to stock with programmer and then you can access it. WARNING, if the truck was programmed with such a tool and it is because it has an egr delete on it, DO NOT return the vehicle to stock. If you do you may not ever be able to flash the pcm with the delete file, because most likely that file doesn't exist and the tuner has been updated to never have that file again. It is possible to do this but not by many, and the few that can, would not let others know it, normally.

7.3 doesn't have an egr but a tuner can cause issues with connections. Try using for
scan if able to see if you can even connect to the pcm.