Petrol driven vehicles contribute to air pollution
because the engine does not completely burn its
air/fuel mixture.
When a petrol engine operates, its exhaust
emissions not only include water and carbon
dioxide, but also noxious emission such as:

CO - Carbon Monoxide - a poison

HC - unburned Hydrocarbons -contributes to smog

NOx - Oxides of Nitrogen

Catalytic converters reduce the output of these three different pollutants, converting them into relatively harmless gases - hence the name « three-way catalyst ».

The use of a 3 way catalytic converter is the most effective pollutant reduction system presently available and enables a reduction in pollutants by approximately 90%+.

The catalytic converter will only operate effectively when the engine is running on a specific air/fuel ratio viz 14.7 parts of air to 1 part fuel. This is known as the Stoichiometric ratio and is tightly controlled through fuel injection, Lambda sensor and electronics.