Various PEMFC operating conditions can lead to irreversible damages as membrane break and gasket deterioration. If leak check procedures can highlight the lack of tightness in fuel cells, they are not appropriate to inform actually where the fault has occurred in the stacks. Thus, diagnosis procedures allowing the failed cells to be identified are needed. Then, these cells can be removed and replaced by safe ones. The article presents some descriptions of aged assemblies subjected to anode/cathode crossover and anode/cooling compartment leakage. Some experimental investigations are carried out with the goal to locate the defective cells. The open current voltages of the cells are monitored and analysed in different operating conditions. Experimental procedures combined with data analysis tools allow the detection of the failed cells, which show abnormal performance and voltage pattern when they are compared to the other safe cells. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.