hello R.Feitor
yes i have been looking through the manual and this system is a lot more sophisticated than the normal JLG systems i am used to seeing. Yes i agree with you that the sensor is most likely located inside the ground control box or even possibly inside the platform control box. In the parts component sections for the base unit and also the platform control station i do not see any references to any type of sensors other than what you indicated by what is indicated in the electronic diagram/schematic. I suspect the sensor is located inside the electronic control.
As for error or warning indicators there is the light on the platform console that will flash MIL codes and also a light on the control board inside the ground control box.
If you need to pull codes from the ECM that can only be done with the handset or the PC version software. If there is a way to get the system to flash codes from memory i don't see any reference to how to accomplish this in the manual. You may have to call JLG and see if their service engineers can give you any pointers on any procedures that are not listed in the manual. Their engineers are good guys and will help if they can, i have dealt with them before in the past and they will help if they can.