Abstract: The composite automatic control system for vehicle engine with theft prevention circuit serves the purpose of controlling the door lock and engine starting power supply. It comprises present correct keys S.sub.1, S.sub.2, S.sub.3 and error keys S.sub.4, S.sub.5. . . S.sub.10. Decoding of the system depends upon the pressing of keys in a sequential order of S.sub.1, S.sub.2, S.sub.3 at predetermined time intervals, for instance 5 seconds as set forth in the model circuit. Failure to do this within the time limit causes the previous input signal to be entirely nullified. The alarm is then triggered to prevent theft upon improper sequence of correct keys or input of error keys.