The need for expert systems in the Heavy Truck and Bus Industry is rapidly approaching a critical point in time. This time reference is the revised emission standards and requirements for heavy duty diesels. This need is related to the influx of electronically controlled engines and their companion the electronically controlled transmission as well as the present shortage of skilled mechanics. This paper discusses the conceptual approach and reasons that expert systems are well suited as a diagnostic aid for vehicular repair. This paper also discusses the implementation of a commercially available expert system for the diagnosis of class 7 and 8 vehicles.The body of this paper will be divided into four discussions. First, applicability of AI/KB programs used for vehicle diagnostics. Second, the use of expert systems to diagnose electronically controlled power modules and their related interactivities with the vehicle. Third, the use of expert systems to diagnose mechanically controlled power modules and their related interactivities with the vehicle. Fourth, the overall advantages of using an expert system to diagnose vehicle failures and the future capabilities of these systems. As a point of clarification, this paper will use the term “power module” to represent the engine-transmission combination and their associated peripheral components. Furthermore, the term “mechanically controlled” will be used to represent engines controlled by governors and transmissions controlled by hydraulic governors and mechanical or pneumatic modulators. It should be noted that expert systems are already in use in such fields as medicine, machine tool, manufacturing and the mechanical design field. As in all ways of life new methods or processes for doing tasks are created on a need basis. With the trend leaning towards “clean-burn” strategy, more and more technology is being forced into heavy duty diesel design. Specifically in the Transit/Coach industry, where some of the most impressive efforts are being made with respect to clean-burn strategy, expert systems will compliment the introduction and continued use of electronically controlled power modules. For the heavy truck and bus Industry this need is upon us and the development of commercially available expert systems has become a reality.