Emission Control System: Purpose
WARNING: This page is about a different car, the 2004 Toyota RAV4. However, it is still accessible from the selected car via links, so may be relevant.
The emission control systems are installed to reduce the amount of CO, HC and NOx exhausted from the engine (3 and 4). They prevent blow-by gas containing HC from being released into the air (1) and evaporated fuel containing HC from being released from the fuel tank (2).
The functions of each system are shown in Fig 1 below: