A combination of electrochemical methods was used to determine failure modes of commercial stacks, run under unknown customer-dependent operational conditions. In this specific case, post-mortem analysis was applied to a commercial 4 kW PEM fuel cell stack. Selected MEAs from this stack were characterized in a test stack by polarization curves, AC impedance spectroscopy and anode and cathode cyclic voltammetry, in order to determine general performance for each cell, ohmic resistance and catalyst layer characteristics. Specific sections from selected MEAs were characterized by the same techniques, in order to determine flowfield dependent variations. It was found that MEAs from the failed stack had increased ohmic resistance together with a strongly reduced anode catalyst surface area, indicating that very fast hydrogen starvation had occurred, causing degradation of the anode catalyst layer. Damage occurred homogeneously over the MEA surface, indicating damage was not related to the flowfield. No degradation was observed at the cathode or the membrane. The combination of electrochemical methods forms a powerful tool to provide detailed insight into cell parameters and degradation for used commercial stacks