Heated Oxygen Sensor (HO2S)
WARNING: This page is about a different car, the 1995 Ford Pickup, 1995 Ford F-Super Duty, and 1995 Ford Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.
The HO2S is mounted in exhaust manifold. When at operating temperature, the HO2S monitors oxygen content of exhaust gases. A heating circuit is used to warm HO2S to operating temperature, enabling faster conversion of feedback system to closed loop operation.
The HO2S produces low voltage (less than .4 volt) to indicate a lean mixture (high amount of oxygen) or high voltage (more than .6 volt) to indicate a rich mixture (low amount of oxygen). This voltage signal is transmitted to PCM.