One of the most effective ways to resolve emissions issues is by sampling the exhaust gases with a five-gas analyzer. Levels of the five gases can indicate the root cause of the problem:
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.