  We study the action of diffeomorphisms on spin foam models. We prove that in
3 dimensions, there is a residual action of the diffeomorphisms that explains
the naive divergences of state sum models. We present the gauge fixing of this
symmetry and show that it explains the original renormalization of
Ponzano-Regge model. We discuss the implication this action of diffeomorphisms
has on higher dimensional spin foam models and especially the finite ones.
