Oxygen Sensor: Description and Operation

Exhaust Oxygen (O2) Sensor:

PURPOSE
The exhaust Oxygen (02) sensor is mounted in the exhaust system where it can monitor the oxygen content of the exhaust gas stream. The oxygen content in the exhaust reacts with the sensor to produce a voltage output. This voltage ranges from approximately 100 mV to 1000 mV.

This voltage must be measured with a digital voltmeter having at least 10 megohms input impedance.

OPERATION
By monitoring the voltage output of the 02 sensor, the Electronic Control Module (ECM) will know what fuel mixture command to give to the injector(s).
Lean mixture - low 02 voltage = rich command.
Rich mixture - high 02 voltage = lean command.

If the 02 sensor circuit is open it should set a Code 13.
A constant low voltage in the sensor O2 circuit should set a Code 44,
while a constant high voltage in the circuit should set a Code 45.

Codes 44 and 45 could also be set as a result of fuel system problems.
Refer to Diagnostic Charts / Diagnostic Trouble Code Charts for conditions that can cause a lean or rich system. Testing and Inspection