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The growth of dilute nitrides with metal organic chemical vapor phase epitaxy (MOVPE) presents two major challenges: a low N-incorporation combined with a high impurity density of carbon and hydrogen. For the device implementation the full understanding of the growth of the metastable material and the influence of the parameters on the impurity levels is of high interest. We investigated the N- and C-incorporation in GaNxAs1-x as a reference system in a close coupled showerhead MOVPE reactor to minimize gas-phase reactions for a wide range of growth parameters and precursors. The N-incorporation was always proportional to the N/As-ratio, while the absolute N-molar-flow, growth rate and interaction time show only a minor influence, indicating little gas-phase pre-reactions. Pyrolysis and pre-reactions between radicals on the surface have been taken into account according to the used precursor combination. We identified the group-III-methyl-groups as the major C-source and found a linear correlation between the incorporated N and C. GaN0.01As0.99 could be grown with C-background below 1016 atoms/cm3 for optimized conditions.