Your auto has a catalytic converter (colloquially, "cat" or "catcon") that is a device used to reduce the toxicity of emissions from an internal combustion engine. 

Catalysis is the process in which the rate of a chemical reaction is either increased or decreased by means of a chemical substance known as a catalyst. [ 1 definition ] Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself.

However, most catalysts will eventually lose their ability to function.

Example of catalyst: [ 2. auto converter ]

Your car has a catalytic converter that is part of the automobile exhaust system. It converts harm­ful compounds in exhaust into harmless compounds. In a typical passenger car, the catalytic converter, which resembles a muffler in shape, is between the engine and the muffler. It's on the underside of the car, usually underneath the passenger seat.

How a Catalytic Converter Works [ 4. How cats work ]

The inside of the catalytic converter is a honeycomb set of passageways or small ceramic beads coated with catalysts: palladium, platinum and rhodium.  These catalysts react in a chemical reaction that makes the gaseous pollutants less harmful.  After the engine exhaust gases pass through the catalytic converter, the new gases go through the muffler and expelled into the air.

Mitsui Mining & Smelting Co. said in April 25, 2008 that it has developed an autocatalyst that replaces platinum with the more affordable silver.  This catalyst is also being implemented in diesel engines as well as gasoline autos.  [ 3. silver converter ]