Abstract: The present invention relates to a diagnostic system for complex systems such as avionics systems. The diagnostic system acquires a description of the system from a variety of design tools and creates a causal network model as an intermediate step. From the causal network model, the diagnostic system builds and compiles a Q-DAG model of the system, which is then embedded in the central maintenance computer of the aircraft. The present invention integrates two elements, a graphical user interface (GUI), which acts as a data capture tool and graphical display of the avionics system and an inference system, which acts as a diagnostic tool with a presenter. The presenter permits diagnosis of faulty sub-systems and a report may be relayed to remotely located maintenance crews to minimize repair time upon arrival of the aircraft.