Front Ho2 Sensor
WARNING: This page is about a different car, the 2004 Kia Rio. However, it is still accessible from the selected car via links, so may be relevant.
The front (upstream) HO2 Sensor is located in the exhaust downpipe after the TWC and provides an input voltage to the ECM. The input tells the ECM the oxygen content of the exhaust gas. The ECM uses this information to fine tune the air/fuel ratio by adjusting injector pulse width.