When you plug MTU USB to CAN diagnostic adapter to your PC,and run MTU DiaSYS diagnostic software,but without communication between PC and MTU USB to CAN diagnostic adapter.It may be a driver error.
MTU USB to CAN Diagnostic Adapter Communication Error:
There is an unknown device and give a “?” mark on the name
So you need to download driver to re-install it,after then you will find the diagnostic adapter to PC communication error solved.
Notice:during driver installation please disconnect MTU Engine USB to CAN diagnostic adapter.
MTU USB to CAN Diagnostic Adapter Driver Download here