As mentioned before, there are two primary catalysts – reduction and oxidation – that can be used within an exhaust system to handle specific gases.

Depending on the year of the vehicle and the type of catalytic converter it has, there might not be a reduction catalyst in place. There are two primary kinds of catalytic converters:

• Two-way: The two-way catalytic converter was present on vehicles in the United States until 1981. They only have oxidation catalysts, which help change carbon monoxide to carbon dioxide. Hydrocarbons (which is unburned and partially burned fuel) are changed to carbon dioxide and water.
• Three-way: Since 1981, the three-way catalytic converter has been used. This performs the same as the two-way converter with the addition of a reduction catalyst. As stated earlier, this is used to change nitrogen oxides to nitrogen and oxygen gases.

Diesel engines employ the use of two-way catalysts, and the converters are also specifically designed to work with diesel exhausts. The converters for these types of engines try and target particulates known as soluble organic fractions. These are made from hydrocarbons bound to soot.