Abstract: A diagnostic system for an engine is provided for which a plurality of annunciators are energized according to particular fault code sequences. A self-diagnosis mode is controlled by use of the vehicle throttle. The self-diagnosis mode can be initiated by a predetermined number of cycles of the vehicle throttle, in which a cycle corresponds to depressing and releasing the throttle beyond upper and lower limits. In one aspect of the invention, a timer is used to determine whether the predetermined number of throttle cycles occurs within a fixed time limit. If not, then the self-diagnosis mode is not activated. Once the self-diagnosis mode is activated, subsequent cycles of the vehicle throttle will increment the annunciators among the sequential fault codes.