  Line-intensity mapping surveys probe large-scale structure through spatial
variations in molecular line emission from a population of unresolved
cosmological sources. Future such surveys of carbon monoxide line emission,
specifically the CO(1-0) line, face potential contamination from a disjoint
population of sources emitting in a hydrogen cyanide emission line, HCN(1-0).
This paper explores the potential range of the strength of HCN emission and its
effect on the CO auto power spectrum, using simulations with an empirical model
of the CO/HCN--halo connection. We find that effects on the observed CO power
spectrum depend on modeling assumptions but are very small for our fiducial
model based on our understanding of the galaxy--halo connection, with the bias
in overall CO detection significance due to HCN expected to be less than 1%.
