Oxygen Sensor: Description and Operation

HEATED OXYGEN SENSOR (HO2S)-II
The heated oxygen senor measures the amount of oxygen present in the exhaust gas. The oxygen sensor "element is coated with a catalyst metal that produces a small voltage from 0 to 1.00 volt at temperatures in the range of 600°F (315°F) when exposed to oxygen. The "stoichiometric" air/fuel mixture for optimum fuel economy and emissions occurs at approximately 0.50 V. When the oxygen sensor output signal is above 0.50 V, the air-fuel mixture is considered rich and when the signal is under 0.50 V it is considered lean.

The OBD II engine management system includes two heated oxygen sensors. (heated oxygen sensors allow the vehicle to go into closed loop in 20 to 30 seconds to reduce emissions)

- The "upstream" (front) oxygen sensor is located in the exhaust stream before the catalytic converter. The front sensor provides feedback to the ECM that is used to identify if the air-fuel mixture supplied to the cylinders is rich or lean. Under normal operating conditions, the front oxygen sensor signal will fluctuate from 0 to 1.00 V. The front sensor signal should fluctuate, crossing the 0.50 V center about seven times in 5 seconds with the engine held at a steady 2500 rpm, when the vehicle is at operating temperature.

NOTE: (The signals are not always visible when viewing with a scan tool because the scan tool's refresh rate is not always fast enough). Hyundai Diagnostic System (HDS) can be used due to a faster "refresh" rate.

- The "downstream" (rear) oxygen sensor is located after the catalytic converter in the exhaust stream. The rear sensor provides feedback to the ECM to "fine tune" the air-fuel ratio and to monitor catalytic converter efficiency. The rear sensor signal will also fluctuate in the 0 to 1.00 V range but the pattern is less predictable than for the front oxygen sensor because of its position in the exhaust system. Once the catalyst has reached operating temperature, the rear sensor signal should change very little, if at all. If the rear sensor signal fluctuates at approximately the frequency of the front sensor, it is an indication that the catalytic converter is not at operating temperature or is malfunctioning.