The three-way catalytic converter is currently the most efficient, safest and most reliable form of exhaust emission control available for petrol engines. As its name indicates, the three-way catalytic converter converts three pollutants, namely hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), into less harmful gases. The catalytic converter is made of a metallic or ceramic substrate material coated with a precious metal (platinum, rhodium, iridium). The catalytic converter converts around 90 percent of the pollutants into carbon dioxide (CO2), nitrogen (N2) and water (H2O). To allow it to perform its task efficiently, the three-way catalytic converter must first reach a certain operating temperature. This is why cars are often fitted with close-coupled preliminary catalytic converters which are able to reduce harmful emissions while the engine is still warming up. The oxygen sensor works in unison with the engine management unit to maintain the composition of the fuel/air mixture at an optimum level for the conversion process.