CO electrooxidation on Pt-based electrodes is simulated by density functional calculations on cluster models. Recently, simple cluster models were used to identify a crucial intermediate species for the reaction. In this work, we address the problem of the initiation of the CO electrooxidation process by employing extended models. These extended models are aimed at representing the surface at full coverage, which is believed to be the condition of the physical system at reaction initiation. According to the models employed in this study, it is concluded that for the reaction to begin at reasonable strength of the potential bias a vacancy has to occur at a site adjacent to an adsorbed CO. Such vacancies can be produced either by the presence of surface defects or due to dynamical desorption of CO.