Nitrogen oxides (NOx) are a group of gaseous pollutants that primarily originate from the combustion of fossil fuels. These emissions contribute to various environmental problems, including acid rain, smog, and respiratory illnesses. Stringent regulations have been implemented to control NOx emission.

When replacing a catalytic converter, it is important to follow the Environmental Protection Agency's (EPA) installation requirements. These requirements are in place to ensure that the new converter is properly installed and will function effectively to reduce emissions.

To appraise the value of a catalytic converter from highest to lowest by make/model, you can follow these steps: 1. Material type. 2. Emission laws. 3. Research the current market value of the precious metals in the catalytic converter. 4. Consider the condition of the catalytic converter. 5. Manufacturing differences.