Carbon dioxide is an unavoidable product of combusting hydrocarbons. Carbon monoxide indicates incomplete combustion and is undesireable, although at least a small amount is unavoidable.
MisterDonut's comment is correct, but I just want to add that it is in practical applications that CO is unavoidable.
If you just look at it on paper, you would think you could avoid it completely. But in practice, there is always some CO. It might be possible to add enough excess air to eliminate it, but not levels that are practical.
