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A new technique for fault diagnosis and estimation of unknown inputs in a class of non-linear systems is presented in this study. The novelty of the approach is based on utilisation of a network of two interconnected sliding mode observers, the first is used for fault diagnosis and the second is used for estimation of unknown inputs. The two observers exchange information about their respective reconstructed signals online and in real time. Conditions and proofs of conversion are presented. A salient feature of the proposed approach is that the state trajectories do not leave the sliding manifold even in presence of unknown disturbances and faults. This allows for faults and unknown inputs to be reconstructed based on information retrieved from the equivalent output error injection signal.