In this paper, the main faults in a commercial proton exchange membrane fuel cell (PEMFC) stack for micro-combined heat and power (m-CHP) application are investigated, with the scope of experimentally identifying fault indicators for diagnosis purposes. The tested faults were reactant starvation (both fuel and oxidant), flooding, drying, CO poisoning, and H 2 S poisoning. Galvanostatic electrochemical impedance spectroscopy (EIS) measurements were recorded between 2 kHz and 0.1 Hz on a commercial stack of 46 cells of a 100-cm2 active area each. The results, obtained through distribution of relaxation time (DRT) analysis of the EIS data, show that characteristic peaks of the DRT and their changes with the different fault intensity levels can be used to extract the features of the tested faults. It was shown that flooding and drying present features on the opposite ends of the frequency spectrum due the effect of drying on the membrane conductivity and the blocking effect of flooding that constricts the reactants' flow. Moreover, it was seen that while the effect of CO poisoning is limited to high frequency processes, above 100 Hz, the effects of H 2 S extend to below 10 Hz. Finally, the performance degradation due to all the tested faults, including H 2 S poisoning, is recoverable to a great extent, implying that condition correction after fault detection can contribute to prolonged lifetime of the fuel cell.