Importance

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Abstract

A control unit 1 has a CPU 101 and a backup RAM 102. The CPU 101 detects diagnostic data necessary for analyzing malfunctions of instruments installed in a motor vehicle, and updates and stores the data in sequence in the backup RAM 102 so that when malfunctions are detected, the CPU 101 inhibits updating and storing of the diagnostic data. Further, the control unit stores the malfunction detection history before the updating and storing inhibiting operation immediately after the malfunction has been detected. Therefore, even if an ignition switch is turned off before the updating and storing inhibiting operation, the malfunction detection history is referenced after the ignition switch is turned on again; when there is a detection history, the updating of the diagnostic data is inhibited so that the diagnostic data is prevented from being reset by mistake when the power supply is turned on again. A CPU 61 of a control unit 51 sets a flag bit at a predetermined position in a RAM 63 when malfunctions of the instruments installed in the motor vehicle are detected and then stores the malfunction code and the diagnostic data. When all the diagnostic data is stored, the flag bit is reset. Since the flag bit is not reset if the power supply is shut off while the diagnostic data is being stored, the diagnostic data can be prevented from being erroneously read out by confirming the setting/non-setting of the flag bit when the diagnostic data is read out.

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First Claim