Program repair has been an active research area for over a decade and has achieved great strides in terms of scalable automated repair tools. In this paper we argue that existing program repair tools lack an important ingredient, which limits their scope and their efficiency: a formal definition of a fault, and a formal characterization of fault removal. To support our conjecture, we consider GenProg, an archetypical program repair tool, and modify it according to our definitions of fault and fault removal; then we show, by means of empirical experiments, the impact that this has on the effectiveness and efficiency of thee tool