Abstract In this study, an analysis of the performance in a commercial Open-Cathode Polymer Electrolyte Fuel Cell (PEFC) stack is conducted using simultaneous Electrochemical Impedance Spectroscopy (EIS) measurements. The factors that limit the performance of the PEFC stack were evaluated with EIS measurements and the impedance model developed in the authors’ previous study. The results showed that adsorbed intermediate species during the oxygen reduction reaction (ORR) limit the PEFC stack performance. The activation overpotential in the stack was dominated by low electrocatalytic activity from one of the cells. The inhomogeneity of air flow rate for this particular PEFC stack played an important role in oxygen transport limitations and ohmic resistance. This study has demonstrated that simultaneous EIS is a powerful tool for in-situ diagnosis of a PEFC stack. This EIS understanding has enabled an assessment of the state of health and performance of the fuel cell stack.