This invention is a computer-based engine-diagnostic method wherein there are provided lists of observable engine faults and causes for each observable engine fault. A user inputs one or more observed engine faults, and the software responds by displaying, in decreasing order by number of occurrences, causes which are common to two or more of the observed engine faults. Thus, the most probable causes for the observed engine faults appear at the head of the display. Additionally, once a probable cause is determined, the software is enabled to display other engine faults which should be observable, given the selected cause.