In a electronic control unit, a computer is operable based on a first power supply voltage output from a power supply circuit. The computer executes a fault diagnostic task. As the fault diagnostic task, the computer controls a timer circuit to change the level of a second activate signal between an inactive level and an active level during an active level of a first activate signal. The computer monitors a level of a monitor signal output from the timer circuit to determine whether the level of the second activate signal is turned depending on the change of the second activate signal by the timer circuit based on the monitored level of the monitor signal.