A diagnostic testing system for use with two or more lighting systems of the High Intensity Discharge (HID) sports lighting type, wherein two or more lighting systems have their ballasts and diagnostic receptacles combined in a single ballast box at the base of a light pole for diagnostic access. Each lighting system has its own diagnostic receptacle electrically coupled to the other lighting system through a common block supplying power to the multi-system fixture. Each lighting system typically comprises at least one lamp, a ballast, a capacitor, and wiring interconnecting the lamp, ballast, and capacitor. Each diagnostic receptacle includes a continuity plug wired to maintain electrical connection to the common block while it is plugged in, and to cause its associated lighting system to be electrically isolated from the common block when the plug is removed. The diagnostic receptacles are adapted to receive a connector from a handheld diagnostic tester which maintains the electrical isolation from the common block during the diagnostic testing.