An interactive diagnostic schematic generator illustrates diagnostic test procedures for diagnosing failure modes of a vehicle. The schematic generator reads a schematic data file such as a wiring diagram from a computer memory, indicates a test subject in the schematic or illustrates a location where test equipment is to be connected, and formats the schematic for display on a display device. The schematic generator also receives feedback from a vehicle onboard computer, test equipment or user input indicating that the action has been performed, and removes the indication of the test subject in the schematic. The schematic generator further marks the test subject to indicate that the action has been performed with a marking style that is visually distinct from the indicating style, optionally indicating that the result has validated or invalidated the test subject.