Oxygen (O2S) / Heated Oxygen (HO2S) Sensor

The upstream Oxygen Sensor is located in the exhaust manifold to monitor the amount of oxygen in the engine exhaust. The measured oxygen level determines the fuel mixture, less oxygen (0.8 volts or higher) indicates a rich fuel mixture while more oxygen (0.2 volts or less) indicates a lean mixture. The ECU constantly cycles between slightly lean and rich to maintain optimal air/fuel ratio. The downstream Oxygen Sensor is located after the catalytic converter and is used to monitor converter efficiency. V6 and V8 dual exhaust engines have additional oxygen sensors for each cylinder bank and after each catalytic converter.

Basic Operation

When an Oxygen Sensor is heated (600 degrees or higher) it will generate an output voltage proportional to the amount of oxygen in the exhaust. The output voltage level is usually between 0.2 to 0.9 volts. The voltage output is not steady. In a properly operating system, the ECU will constantly cause the output voltage to swing above and below the midrange point of 0.45 volts, therefore, it will be difficult to measure the output using a digital voltmeter. The output resembles an AC voltage pattern of about 0.3 to 0.7 volts peak-to-peak.