The Engine Control Unit (ECU) monitors the signals of input and output sensors, some all the time and others at certain times and processes each signal. When the ECU has noticed an irregularity has continued for a specified time or longer from when the irregular signal was initially monitored, the ECU judges that a malfunction has occurred and will memorize the malfunction code. The code is then stored in the memory of the ECU and is accessible through the data link (diagnostic connector) with the use of an electronic scan tool or a voltmeter.