The system consists of a pump, the air cut-off valve and the solenoid valve and is controlled by the ecu. When the engine is cold , the engine solenoid valve switches the air valve to open and the pump injects a controlled amount of air into the cylinder head exhaust ports.The air then mixes with the exhaust gases , Causing any unburned particles of the fuel in the mixture to be burnt in the exhaust manifold which effecttively raises the tempreture of the exhaust gasses. Once the catalytic convertor is up to tempreture the solenoid valve closes the air valve and the pump is switched off. A non-return valve prevents the exhaust gasses passing through the air valve.