The GM-49903-5 Diplexer Combiner supports a composite of cell phone and gap signals on a single coax input for the diagnosis of On-Star Gen9 and Gen10 systems. It is used with the GM-49903-1 Substitute Antenna, GM-49903-2 Adapter Cable and GM-49903-4 Gen9 Adapter Cable to allow connection to more than one type of On-Star module. This will determine if the problem is with the vehicle antenna circuit or the On-Star module itself.