A vehicle diagnostic system capable of diagnosing ECUs for partial networking includes a communication bus mounted on a vehicle, a plurality of controllers for controlling vehicle operations, and a diagnostic equipment to collect diagnostic information from the controllers through the communication bus and to diagnose the vehicle operations. The controllers are connected with each other through the communication bus. Each of controllers operates in a normal mode and in a sleep mode, the sleep mode being an operation mode in which all of or a part of the functions excluding at least a function for receiving an wake-up instruction to return to the normal mode through the communication bus are suspended. At least one of the controllers is configured to send the wake-up instruction to all the other controllers in response to receiving a predetermined signal from the diagnostic equipment.