O2S Sensors
WARNING: This page is about a different car, the 2005 Suzuki Reno and 2005 Suzuki Forenza. However, it is still accessible from the selected car via links, so may be relevant.
The O2S sensor is located in the exhaust manifold. The O2S sensor indicates to the ECM the amount of oxygen in the exhaust gas and the ECM changes the air/fuel ratio to the engine by controlling the fuel injectors. The best air/fuel ratio to minimize exhaust emissions is 14.7 to 1, which allows the catalytic converter to operate most efficiently. Because of the constant measuring and adjusting of the air/fuel ratio, the fuel injection system is called a "closed loop" system.
The ECM uses voltage inputs from several sensors to determine how much fuel to provide to the engine. The fuel is delivered under one of several conditions, called "modes."