How is carbon monoxide formed in a car engine?

Answer 1

When the car is idling in an enclosed space, such as a garage, the Oxygen in the enclosed space is constantly being used up by the engine to burn the fuel. In time, the amount of Oxygen in the air decreases and the amount of Carbon-dioxide increases. So after a while, the engine does not get enough Oxygen to burn the fuel completely resulting in incomplete combustion of fuel. This incomplete combustion results in the production of Carbon-monoxide (CO) instead of Carbon-dioxide (CO2).

Similarly when the tailpipe of an automobile is obstructed (by water or other physical obstructions), this creates a back-pressure to the engine and the engine does not suck in enough air from the atmosphere resulting in incomplete combustion of fuel followed by production of Carbon-monoxide.