Patent ID: 7518261

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, a state of an electric load connected to said electronic control unit and supplied with electric power from said vehicle battery when said main relay is in the on state after said control section outputs said stop command; and diagnosing whether or not said main relay is in a fault state where said main relay cannot be controlled from the on state to the off state on the basis of monitored state of said electric load.