What are the sources of carbon monoxide?
Incomplete combustion of hydrocarbons.
The internal combustion engines is tuned so that some of the hydrocarbon is combusted to carbon or carbon monoxide. Why? I am not an engineer (but would like to talk to one!), but I assume that optimum performance, and optimum engine life is achieved when hydrocarbons are not combusted completely. Certainly car exhausts produce both particulate carbon, and carbon monoxide, the products of incomplete combustion.
We could represent this incomplete combustion for hexanes: