Fault detection method

The common power supply terminal (13) is connected to a common power supply (11) mounted on a vehicle. The starting power supply terminal (23) is connected to the starting power supply (21) via a starting switch (22) that is turned on when the vehicle is started. The CPU (40) is activated by the supply of a common voltage input through the common power supply terminal (13). The wake-up circuit (30) has a wake-up switch (32) that opens and closes a power supply path from the common power supply terminal (13) to the CPU (40), and turns on the wake-up switch (32) when a start-up voltage input to the start-up power supply terminal (23) is equal to or greater than an ON threshold value. When the normal voltage is equal to or greater than the determination threshold value, although the starting voltage is less than the ON threshold value, the CPU (40) determines that an ON fixed failure of the wake-up switch (32) has occurred. 常用电源端子(13)与搭载于车辆的常用电源(11)连接。起动用电源端子(23)经由在车辆起动时被进行接通操作的起动开关(22)与起动用电源(21)连接。CPU(40)通过经由常用