The Heated Oxygen Sensor (HO2S) (Figure 33) detects the presence of oxygen in the exhaust and produces a variable voltage according to the amount of oxygen detected. A high concentration of oxygen (lean air/fuel ratio) in the exhaust produces a low voltage signal less than 0.4 volt . A low concentration of oxygen (rich air/fuel ratio) produces a high voltage signal greater than 0.6 volt . The HO2S provides feedback to the PCM indicating air/fuel ratio in order to achieve a near stoichiometric air/fuel ratio of 14.7:1 during closed loop engine operation. The HO2S generates a voltage between 0.0 and 1.1 volts .
Embedded with the sensing element is the HO2S heater. The heating element heats the sensor to temperatures of 800°C (1400°F) . At approximately 300°C (600°F) the engine can enter closed loop operation. The Vehicle Power (VPWR) circuit supplies voltage to the heater and the PCM will complete the ground when the proper conditions occur. For model year 1998 a new HO2S heater and heater control system are installed on some vehicles. The high power heater reaches closed loop fuel control temperatures. The use of this heater requires that the HO2S heater control be duty cycled, to prevent damage to the heater. The 6 ohm design is not interchangeable with new style 3.3 ohm heater.