This paper presents the FunctionalDiagnosis with efforts and flows (FDef) approach to model-baseddiagnosis. First, it motivates our research by considering the field of functionalflow-based diagnostic techniques, pointing out some of their relevant limitations, which are overcome by FDef. Then, it summarizes and exemplifies the main FDef concepts and techniques in a tutorial way. A real-world diagnostic problem in the marine engineering domain has been used to evaluate the basic FDef engine. The paper summarizes the results of the evaluation and discusses the extensions it has prompted, i.e., the introduction of qualitative deviations of variables and the exploitation of a hierarchical representation of the functional model. The ideas proposed in FDef can be easily adapted to other flow-based modeling formalisms, such as MFM.
