Abstract: A circuit testing device adapted and operable to allow a single technician to properly and practically test an electrical subsystem of a vehicle. More specifically, the device is adapted to substantially automatically perform the particular recommended or required tests for investigating or diagnosing the electrical subsystem, and to straightforwardly communicate the results of those tests as being either satisfactory or unsatisfactory without requiring complex interpretation by the technician. The device broadly comprises a housing, including a display and control panel; one or more connectors; and testing circuitry. The display and control panel uses green LEDs to indicate satisfactory or good results and red LEDs to indicate unsatisfactory or bad results.