“Fault tree” is a name given to a logic diagram that develops all of the subsystem and component faults and combinations of faults which can result in particular system symptoms or faults. This type of logic diagram can be extremely useful in all phases of automobile design and service. Applications are discussed to the following areas: 1. As a reliability tool for identifying and cataloging specific problems, to preclude their being incorporated in new designs. 2. As a diagnostic aid to maintenance personnel in systematically screening potential vehicle performance problems. 3. As an aid in assessing accident causation factors and the potential contribution of vehicle defects to accidents. Examples in each area are presented. Particular emphasis is placed on the value of such logic diagrams over conventional troubleshooting charts and manuals in aiding mechanics to pinpoint specific problems. Problems and pitfalls in developing such detailed logic diagrams and implementing the results in a field service environment are also discussed.