NOTE: Some late model vehicles utilize two heated oxygen sensors. One mounted in the exhaust manifold, and one threaded into the exhaust pipe.
The heated oxygen sensor, mounted in the exhaust gas flow, is a device which produces an electrical voltage when exposed to oxygen present in the engine exhaust gases. The electrical voltage produced by the oxygen sensor is proportional to the amount of oxygen present in the exhaust and therefore, representative of the air/fuel ratio. The voltage produced by the sensor is sent to the ECM. The ECM will check whether the actual air/fuel mixture ratio is richer or leaner than the optimal (theoretical) ratio, and adjust accordingly. The heated oxygen sensor is electrically heated internally for faster switching during cold engine operation.
If the oxygen sensor has malfunctioned, the driveability of the vehicle may not be influenced. However, hazardous components (HC, CO, NOx) in the exhaust gas will increase.