An engine's combustion efficiency can be analyzed by measuring the levels of five different gases in the exhaust stream. The five gases we monitor are HC, NOx , CO, CO2 , and O2 . Carbon dioxide or CO2 is the product of perfect combustion. Unfortunately, even CO2 is considered a greenhouse gas. The greenhouse gases are carbon dioxide, nitrous oxide, methane, and chlorofluorocarbons.
By monitoring these five gases, we have an idea of what's going on inside, and sometimes outside of the combustion chamber. Three of these are considered pollutants; HC's, NOx , and CO.
HC's or hydrocarbons are leftover, unburned fuel found in the cylinder after incomplete combustion. All engines have a little excess fuel left over after ignition; the question is how much is acceptable. Rich air-fuel mixtures result in elevated HC's because there's too much fuel in the mixture. Too lean of an air-fuel ratio will also cause excessive HC's in exhaust emissions. This is called lean burn and results from the misfire caused by insufficient fuel in the mixture. Unburned fuel entering the catalytic converter overheats and damages the substrate.
NOx High temperatures in the combustion chamber cause NOx or oxides of nitrogen. Oxygen and nitrogen combine in the combustion chamber at temperatures of 2500° F or higher to create this pollutant. Both oxygen and nitrogen enter the combustion chamber; the key is to keep cylinder temperatures below this crucial 2500° F to prevent the production of this harmful gas. NOx is considered a pollutant and causes smog. Anything that results in higher than normal cylinder temperatures like an overheating engine, faulty EGR valve, and lean air fuel ratios aid in the creation of NOx.
CO or carbon monoxide is considered a harmful gas and byproduct combustion. It occurs when carbon and oxygen combine. When there's too much fuel in the mixture, and the air-fuel ratio is rich. When an engine is running correctly, stoichiometric ratios are achieved, and CO levels are low. High CO levels are an indicator of a rich air-fuel ratio. This could be caused by a leaking fuel injector or high fuel pressure. Also, make sure nothing is restricting the airflow, like a clogged air filter or plugged PCV system.