  Carbon monoxide (CO) emission has been observed in a number of core-collapse
supernovae (SNe) and is known to be an important coolant at late times. We have
implemented a chemical reaction network in the radiative-transfer code CMFGEN
to investigate the formation of CO and its impact on SN ejecta. We calculate
two 1D SN models with and without CO: a BSG explosion model at one nebular
epoch and a full time sequence (50 to 300 days) for a RSG explosion. In both
models, CO forms at nebular times in the dense, inner regions at velocities
$<2000 \mathrm{km/s}$ where line emission from CO can dominate the cooling and
reduce the local temperature by as much as a factor of two, weakening emission
lines and causing the optical light curve to fade faster. That energy is
instead emitted in CO bands, primarily the fundamental band at $\sim
4.5\mathrm{\mu m}$, which accounts for up to 20% of the total luminosity at
late times. However, the non-monotonic nature of the CO cooling function can
cause numerical difficulties and introduce multiple temperature solutions. This
issue is compounded by the sensitivity of the CO abundance to a few reaction
rates, many of which have large uncertainties or disparate values across
literature sources. Our results also suggest that, in many SNe, CO level
populations are far from their LTE values. Unfortunately, accurate collisional
data, necessary to compute NLTE populations, are limited to a few transitions.
