This paper presents a detailed procedure for automatic supervision, fault detection, and diagnosis of possible failure sources leading to total or partial loss of productivity in grid connected PV systems. The diagnostic procedure is part of the monitoring system allowing, at the same time, modeling and simulation of the whole system and variables measurements in real time. The fault detection algorithm is based on the comparison of simulated and measured yields by analyzing the losses present in the system while the identification of the kind of fault is carried out by analyzing and comparing the amount of errors deviation of both DC current and voltage with respect to a set of errors thresholds evaluated on the basis of free fault system. The proposed method has been validated in with experimental data in a grid connected PV system in the Centre de Developpement des Energies Renouvelables (CDER) in Algeria. (C) 2013 Elsevier Ltd. All rights reserved.