Abstract A large amount of hydrogen can be released into the containment of light water reactors during a severe accident. Passive Auto-catalytic Recombiners (PARs) aim to avoid flame acceleration and excessive pressure loads on the containment in case of hydrogen combustion. Their operation is based on the catalytic recombination of hydrogen into steam in the presence of oxygen. Thus, the recombiners reduce the hydrogen but also the oxygen content in the containment atmosphere. As a consequence, the oxygen/nitrogen ratio diverts more and more from the standard 21 vol.% in air. This decreasing ratio may impact on the PAR efficiency. Additionally, the exothermic surface chemical mechanism leads to the overheating of the catalytic plates and activates the natural convection inside the recombiners. This heat source can also create local conditions for hydrogen combustion in the gas phase, as igniters do. Hence, the oxygen/nitrogen ratio may also determine the conditions for the gas-phase ignition inside PARs. This study deals with the numerical simulation of the impact of the oxygen starvation (i.e. low oxygen/nitrogen ratio) on the PAR efficiency and on the PAR gas-phase ignition limit. Calculations are performed with a dedicated CFD code named SPARK. We focus on the interaction of recombiners with any H2/O2/N2/H2O mixtures and thus establish a quite complete understanding of PAR operation. Calculations confirm the experimental oxygen surplus (i.e. twice more oxygen than stoichiometry) necessary to ensure an optimal PAR efficiency ( X O 2 ≈ X H 2 ) independently of the steam content. The PAR gas-phase ignition limit is then determined numerically in the classical H2/Air/H2O ternary diagram with a very good agreement with the available experimental database. It points out the importance of catalyst heat radiation, and more secondarily of species thermal diffusion (i.e. Soret effect). Finally, the PAR gas-phase ignition limit is determined for all oxygen/nitrogen ratios. The ignition domain appears to strongly contract when the oxygen content decreases, so that the steam threshold for inertization of the containment with respect to the recombiners ignition becomes very low.