The (O2) Oxygen Sensors is often referred to as the “O2” sensor because O2 is the chemical formula for oxygen.

So, Monitoring (O2) Oxygen Sensors levels in the exhaust is a way of gauging the fuel mixture.

Consequently, the (O2) Oxygen Sensors tells the computer if the fuel mixture is burning rich (less oxygen) or lean (more oxygen).

Oxygen Sensors
Oxygen Sensors

The OBD II system compares the oxygen level readings of the (O2) sensor before and after the converter to see if the converter is reducing the pollutants in the exhaust. Therefore, If it sees little or no change in oxygen level readings, it means the converter is not working properly. This will cause the Malfunction Indicator Lamp (MIL) to come on.

As a result, there are alot of factors can affect the relative richness or leanness of the fuel mixture:

  • Air temperature
  • Engine coolant temperature
  • Barometric pressure
  • Throttle position
  • Air flow
  • Engine load

Consequently, any problems with the O2 sensor can throw the whole system out of whack.

The O2 sensor actually generates electricity based on the output of the exhaust system. The computer then senses this variance in voltage making adjustments to the fuel mixture. The voltage produced ranges from 0.9 down to 0.1 volts. This will tell the computer the mixture is either too lean or too rich. Therefore, The higher the voltage is, the richer the exhaust from the engine is and this information allows the car to lean the mixture out.

Also, Sometimes the voltage output from the sensor can fail or the variance in voltage can become sluggish, not responding fast enough for the computer. When the sensor fails the computer is unable to perform key adjustments to allow the engine to perform efficiently. Identifying which sensor has failed can be tricky and most times it requires the use of advanced scanning tools.

Vehicles with multiple sensors are monitoring the performance of the catalytic converter as well. If a sensor is located before the catalytic converter its job is to adjust the mixture of the fuel. Rather,  the sensor after is monitoring the catalytic converters performance and efficiency.

The first thing we need to find is the Fault Code for the rich or lean condition ?

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(O2) Oxygen Sensor Trouble Codes:


Any O2 sensor that is defective obviously needs to be replaced. But there may also be benefits to replacing the O2 sensor periodically for preventive maintenance. Replacing an aging O2 sensor that has become sluggish can restore peak fuel efficiency. This will minimize exhaust emissions and prolong the life of the converter.

So, If an O2 sensor is marginally sluggish or is slightly biased rich or lean, it may not set a fault code. Consequently, The only way to know if the O2 sensor is functioning normally is to check its responsiveness to changes in the air/fuel mixture. You can read the O2 sensor voltage output with a scan tool or digital voltmeter, but the transitions are hard to see because the numbers jump around so much.

In conclusion, The best way to observe O2 sensor output voltage changes is with a Digital Storage Oscilloscope (DSO). A scope will display the sensor voltage output as a wavy line that shows both it’s amplitude (minimum and maximum voltage) as well as its frequency (transition rate from rich to lean).