The problem of fault detection and isolation (FDI) is investigated for a class of non-linear systems and applied to the case of the free radical copolymerization of butyl acrylate (BuA) and vinyl acetate (VAc) in an ethyl acetate (EAc) solvent. The approach proposed in this work is based on decoupling techniques using differential geometry theory and observer synthesis for non-linear systems. A design procedure is provided for residual generation. The faults considered in the application are malfunctions of the feed pumps of both monomers and the initiator, and the presence of an inhibitor. A detailed construction of fault detection filters is presented, and their performances in the presence of parameter uncertainties are discussed through some simulations.