  We use spatially resolved scanning tunneling spectroscopy in Na(Fe{1-x}Cox)As
to investigate the impurity effect induced by Co dopants. The Co impurities are
successfully identified, and the spatial distributions of local density of
state at different energies around these impurities are investigated. It is
found that the spectrum shows negligible spatial variation at different
positions near the Co impurity, although there is a continuum of the in-gap
states which lifts the zero-bias conductance to a finite value. Our results put
constraints on the S+- and S++ models and sharpen the debate on the role of
scattering potentials induced by the Co dopants.
