Membrane dehydration, fuel cell flooding and anode catalyst poisoning by carbon monoxide are diagnosed in a single polymer electrolyte fuel cell (PEMFC) using nonlinear frequency response analysis (NFRA). A sinusoidal perturbation of high amplitude is applied to the fuel cell current and the resulting voltage is analysed using the concept of higher order frequency response functions. It is shown that the linear part of the system response corresponds to classical Electrochemical Impedance Spectra (EIS), which are not sufficient to clearly distinguish between different fuel cell failures. Therefore, the nonlinear behaviour is additionally taken into account in the form of the second order frequency response function. With this, it is possible to distinguish unambiguously between the three analysed PEM fuel cell failures. (C) 2009 Elsevier B.V. All rights reserved.