Expert systems are becoming increasingly important in the application of artificial intelligence technology to real-world problems. An expert system is defined as a computer program which models the decision making process of an expert. This paper explores the concept using expert systems as external diagnostic aids in the repair of motor vehicles. The applicability of expert systems to vehicle diagnosis is first described. The paper then discusses general principles to be considered when building expert systems for vehicle diagnosis. The concept of using expert systems for repairing electronically controlled engine systems is explored in greater detail, and a discussion is provided on a prototype expert system to perform this type of diagnosis. Recommendations are provided on what would be required to develop a commercial expert system based upon the prototype expert system discussed in this paper.