Heated Oxygen Sensor

Heated Oxygen Sensors (HO2S) are mounted in or right after exhaust manifold before catalytic converter (upstream HO2S), and in exhaust pipe after catalytic converter (downstream HO2S). HO2S detects presence of oxygen in exhaust gases and produces a variable voltage according to amount of oxygen detected. A high concentration of oxygen (lean air/fuel ratio) in exhaust gases produces a low voltage signal less than 0.4 volt. A low concentration of oxygen (rich air/fuel ratio) produces a high voltage signal more than 0.6 volt. HO2S provides feedback to PCM indicating air/fuel ratio in order to achieve a near stoichiometric air/fuel ratio of 14.7:1 during closed loop engine operation. HO2S generates a voltage between 0.0 and 1.1 volts.

Embedded with the sensing element is the HO2S heater. The heating element heats sensor to 1400°F (800°C). At approximately 600°F (300°C), engine can enter closed loop operation. VPWR circuit supplies voltage to heater and PCM will complete the ground when the proper conditions occur. Starting in 1998, a new HO2S heater and heater control system are installed on some vehicles. The high power heater reaches closed loop fuel control temperatures. Use of this heater requires HO2S heater control be duty cycled, to prevent damage to heater. The 6-ohm design is not interchangeable with new style 3.3-ohm heater.