Oxygen (O2) Sensor
WARNING: This page is about a different car, the 1993 Toyota Celica. However, it is still accessible from the selected car via links, so may be relevant.
O2 sensor monitors exhaust gas oxygen content and delivers an input signal to Engine Control Module (ECM). The ECM uses input signal to determine fuel injection system operation. Some models may be equipped with more than one oxygen sensor and a sub-oxygen sensor. Some models may contain a heater to warm the oxygen sensor.