Front Oxygen Sensor (02S)

The front oxygen (02S) sensor is mounted in the exhaust system where it can monitor the oxygen content of the exhaust gas stream. An oxygen sensor acts as an electrochemical cell, or battery, to produce a voltage in response to the oxygen content in the exhaust stream compared to the oxygen content of the atmosphere. A lean mixture (high oxygen content in the exhaust) will produce a low voltage and a rich mixture (low oxygen content in the exhaust) will produce a high voltage. The oxygen content in the exhaust reacts with the oxygen to produce a voltage output. This voltage ranges from approximately 100 millivolts (0.1 volts) (high-oxygen lean mixture) to 900 millivolts (0.9 volts) (low oxygen-rich mixture).
By monitoring the voltage output of the oxygen sensor, the PCM/EC will know what fuel mixture command to give to the injector (lean mixture - low voltage - rich command; rich mixture - high voltage - lean command).

IMPORTANT: A DTC 44 or 45 usually indicates a rich or lean engine operation, rather than a defective oxygen sensor or circuit. Therefore, it is important that the Fuel System Check be used to diagnose these codes.