When hydrogen leaks through holes in membrane-electrode assemblies (MEAs) in PEM fuel cells, it recombines directly with air. This recombination results in a reduction in oxygen concentration on the cathode side of the MEA. In this book, the signatures of electrochemical impedance spectroscopy (EIS) are measured and analyzed in a single and different multi-cell stack configurations to show the relation between hydrogen leak rate and reduced oxygen concentrations. A model that establishes a relationship between impedance and voltage was considered. Using this model along with the impedance signatures we were able to detect the reduction of oxygen concentrations at the cathode with the help of fuzzy rule-base.