Fuel cell systems are complex systems and a high degree of competence is needed in different areas of knowledge such as thermodynamics, fluid dynamics, electrochemistry, and others, for their comprehension. This paper is a contribution to global modeling and fault diagnosis of these systems. More precisely, the goal of this paper is twofold. First, an electrical equivalent model, which could be used as a unifying approach to fuel cell systems will be resented. Second, a methodology to use the electrical model for fuel cell system diagnosis will be introduced, making special emphasis on fuel cell flooding detection. In order to illustrate the relevance of the proposed approach, experimental validations of the model, and the diagnosis methodology are proposed.