Automotive Emissions and Pollutants
There are five gases we automotive technicians monitor as an indication of combustion efficiency. 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. 
The five gases we monitor in a vehicles emissions are HC, NOx, CO, CO2, and O2. By monitoring these five gases we know what is going on inside and sometimes outside the combustion chamber. Of the five monitored gases, the three that are considered pollutants are HC's, NOx, and CO. Let's have a look.
HC's or hydrocarbons are left over unburned fuel left in the cylinder by incomplete combustion. All engines have a little excess fuel left after ignition in the combustion chamber, the question is how much is acceptable. A rich air fuel mixture will cause elevated HC's because there is 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. A rich fuel mixture or misfire will result in excess unburned fuel (HC's) to enter the catalytic converter resulting in a damaging overheating condition.
Anything that causes too rich or too lean a fuel mixture will cause excess HC's. The key is to maintain the mixtures stoichiometric or 14.7 to 1 air to fuel mixture. This indicates 14.7 parts of air for every 1 part of fuel. A great indicator and the product of good combustion is higher levels of CO2 not HC.
NOx or oxides of nitrogen are the result of high temperatures in the combustion chamber.
Oxygen and nitrogen combine in the combustion chamber and with a temperature of 2500° F or greater to create this pollutant. Since 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 gas. NOx is considered a pollutant and causes smog. Anything that causes high cylinder temperatures like an overheating engine, faulty EGR valve, or lean fuel mixtures aid in the creation of NOx.
CO or carbon monoxide is considered a harmful gas and byproduct of a combustion engine. It is the combination of carbon and oxygen and occurs when there is not enough oxygen to combine with the fuel during combustion. When an engine is running correctly and when stoichiometric ratios are achieved CO levels are low. When the air fuel ratio is rich the CO levels rise. This is why high CO levels are a great indicator of a rich air fuel ratio. When diagnosing high levels of CO look for anything that causes a rich fuel mixture like a leaking fuel injector or high fuel pressure. Also make sure nothing is restricting the air flow like a clogged air filter or plugged PCV system.
