Heated Oxygen Sensor 2 (HO2S-2)

The HO2S-2 is located behind the main catalytic converter. The function of the sensor is to monitor the efficiency of the catalytic converter. The sensor is a four-wire sensor; two are used for the heater and the other two are used for the oxygen sensor signal. The HO2S-2 is an electrical source that responds to oxygen content in the catalytic converter. When the sensor reaches approximately 316°C (600°F), it produces a voltage based on the difference in oxygen between the atmosphere and exhaust gas. The PCM sends a bias voltage (391 - 491 mV) on the signal line which can be read by the Scan tool when the sensor is cold. When the HO2S-2 is cold, it produces no voltage and has extremely high internal resistance. However, when the sensor heats up, it produces voltage that overrides the bias voltage. This voltage is read by the PCM to determine the efficiency of the catalytic converter. Under normal conditions, low sensor voltage means high oxygen content/lean air-fuel mixture and vice versa. Normal sensor readings at steady cruise with a properly functioning catalytic converter will be steady around 500 - 800 mV.