Abstract: A computer program architecture for a motor vehicle on-board diagnostic system includes a plurality of monitor modules for monitoring vehicle systems or components and issuing a malfunction subroutine call to a diagnostic executive upon detecting a system or component malfunction. The executive includes a plurality of software objects or modules for carrying out malfunction indicator light control and fault code storage strategies. A diagnostic scheduler module implemented as a finite state machine, controls and coordinates the sequence of the test to be run by the monitor modules as well as on-demand self tests. A malfunction indicator light (MIL) control module implements four distinct light control strategies through finite machines for illuminating and extinguishing the light and for storing and erasing fault codes under predetermined conditions.