We know that gasoline itself contains a wide array of hydrocarbons .So the presence of hydrocarbons in the eahaust most likely indicates that some of the gas has made it through the engine and into the exhaust stream without burning. This could mean that some hydrocarbon molecules have simply not reacted at all before leaving the engine. Or it could mean that some larger molecular have made it into the exhaust. What about the presence of carbon monoxide? We wrote our original quation based on the assumption of complete combustion. This means that all carbon atoms combustion will be incomplete, and CO is produced instead of CO2