Patent ID: 7446428

Claim:
A method of diagnosing presence of a fault in a main relay by use of an electronic control unit including a control section monitoring a manipulation of an ignition switch of a vehicle, and a timer section for automatically starting up said control section, said electronic control unit being configured to control said main relay to an on state for supplying said control section with electric power from a vehicle battery when said ignition switch is turned on or when said timer section in which a predetermined start-up set time is set is timed up, and to control said main relay to an off state in accordance with a stop command outputted from said control section, said method comprising the steps of: monitoring, by use of said control section being supplied with electric power through said main relay, a time elapsed since said control section outputs said stop command; and diagnosing that said main relay is in a fault state where said main relay cannot be controlled to the off state when said monitored time exceeds a threshold time predetermined on the basis of a response time needed for said main relay to change from the on state to the off state in response to said stop command.