A catalytic converter, or "cat," is an emissions control device in your car's exhaust system. Its job is to reduce the harmful pollutants in engine exhaust—specifically hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx)—by converting them into less harmful gases like water vapor, carbon dioxide, and nitrogen through chemical reactions. It's a key component for keeping a vehicle compliant with environmental regulations.
Inside the stainless steel casing, you'll find a honeycomb-style ceramic block or substrate. This structure is coated with precious metals like platinum, palladium, and rhodium, which act as catalysts. As hot exhaust gases pass over these metals, a chemical reaction occurs without the catalysts themselves being consumed. This process is called catalysis.
There are two main types. The most common is the three-way catalytic converter, which handles all three major pollutants simultaneously. Diesel engines often use an oxidation catalytic converter (DOC) that primarily targets HC and CO.
The converter relies on data from oxygen sensors before and after the unit to function efficiently. If it fails, you'll often see a check engine light and may experience reduced performance, a rotten egg smell from the exhaust, or failed emissions testing. Replacement is expensive due to the valuable metals inside, making catalytic converters a frequent target for theft.
The effectiveness of a modern catalytic converter is significant, as shown by the reduction in tailpipe emissions since their introduction.
| Pollutant Type | Before Catalytic Converter (g/mile) | After Catalytic Converter (g/mile) | Average Reduction |
|---|
| Hydrocarbons (HC) | 3.4 - 4.1 | 0.25 - 0.41 | ~90% |
| Carbon Monoxide (CO) | 34 - 41 | 3.4 - 4.1 | ~90% |
| Nitrogen Oxides (NOx) | 3.0 - 4.0 | 0.4 - 1.0 | ~75-90% |
| Particulate Matter (PM) | 0.06 - 0.12 | 0.01 - 0.02 | ~80-90% |
