Of the many molecules and compounds coming out of the tailpipe, the EPA is primarily concerned with five of them:
Carbon Monoxide (CO)
Oxides of Nitrogen (NOx)
Carbon Dioxide (CO2)
One of the most effective ways to resolve emissions issues is by sampling the exhaust gases:
High HC emissions indicate unburned fuel.
High CO levels indicate partially burnt fuel or oil.
High NOx levels are normally caused by high combustion temperatures and pressures, slightly lean AFR, and excessively advanced ignition timing.
Tailpipe emissions readings low in HC and CO levels with high NOx emissions are typically NOT caused by a defective converter. The low HC and CO readings indicate that the converter is functioning. The root cause of the problem is an engine which is emitting excessively high NOx emissions. These high NOx emissions may reduce the durability and efficiency of the converter.