The poor durability of proton exchange membrane fuel cells still is their greatest barrier preventing commercialization. In this paper, we perform a 1465 h in situ durability test with a single fuel cell operating at a constant-current of 800mA cm(-2). Polarization curves, cyclic voltammetry, and electrochemical impedance spectroscopy are used for diagnostics and to understand changes in performance during the test. Limiting current method is used to analyze the oxygen transport resistance of the gas diffusion layer (GDL) before and after durability test. The results indicate that the oxygen transport resistance in single cell increases greatly, and GDL is the first component leading to fuel cell failure after 1465h test. At 150 kPa and 20% dry oxygen mole fraction, the limiting current decreases to about 700mA cm(-2) and total oxygen transport resistance increases by 0.1623s cm(-1), implying more serious flooding occurs in GDL after the 1465 h durability test. The increase of oxygen transport resistances of the substrate and micro porous layer of GDL confirm the loss of water management capability of GDL. SEM images show that these may be caused by oxidation of carbon and loss of polytetrafluoroethylene in GDL. (C) 2019 The Electrochemical Society.