Polymer electrolyte membrane fuel cells are devices that produce power by direct conversion of hydrogen via electrochemical route and are promising for energy applications, mainly because no direct pollutants are produced during operation. Automotive is the major industrial application for polymer fuel cells, which could replace internal combustion engines as power sources, conditionally to the achievement of a significant cost reduction. Increasing power density and reducing the loading of precious metal based catalysts is thus a technological priority. In this direction, the geometry of the flow field plays a dramatic role: at state of the art, hydrogen and oxygen are distributed over the fuel cell area through channels. Non-uniform distribution of reactants, which results from non-optimal flow field design, determines heterogeneity during operation, loss of efficiency and accelerates ageing. In this work, computational fluid dynamics is used to analyse oxygen transport in a low platinum polymer electrolyte fuel cell for automotive applications. Analysis focuses on the effect of 3D geometrical features that are present in state of the art flow fields. Comparison of three flow fields (straight channel, serpentine and interdigitated) is performed and it is observed that the contact points between the GDL and the current collector determine significant performance loss because of sluggish oxygen transport in these regions. Nevertheless, a trade off with electron transport through the GDL must be considered. To support the conclusions of the work, an original methodology is adopted, by simulating electrochemical impedance spectroscopy, an experimental transient technique that allows to selectively evidence the effect of mass transport from other physical phenomena.