In order to increase the reliability of fuel cells, an online diagnostic method for detection of operation malfunctions, as well as the early detection of failures in the fuel cells, is necessary. For this purpose, locally resolved current density measurements are an important tool, but the interpretation of the data related to the detection of malfunctions or failures is not straightforward. Here, segmented cell technology is applied to investigate the current density distributions in the anode and cathode electrodes to ascertain their equivalence due to the strong perpendicular coupling of currents. Current density distributions are further used to determine the signature of pinhole formation in the membrane. Different behavior is observed for membrane leakage under open circuit and under applied load conditions. Whereas the cell at open circuit is characterized by a positive current in the vicinity of the pinhole and small negative currents in the remaining area, an applied load leads to large negative currents at the pinhole. The characteristic behavior can be explained by high crossover rates of hydrogen from the anode to the cathode. The nongeneric signature is used to detect the deterioration of a membrane electrode assembly after a test stand malfunction. A sudden pressure drop associated with vaporation of water and the fast cooling of the cell is assumed to trigger the failure of the membrane. After 48 h, fissures in several positions of the membrane near the edges of the cell holder are observed. Through the evolution of leakages in the fuel cell, a malfunction can be detected at an early stage and thereby catastrophic failure of the whole stack may be avoided or anticipated. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3504255] All rights reserved.