The number of 02 sensors your vehicle has depends on its year, make,
and model. Modern vehicles have an oxygen sensor before and after
the catalytic converter. Some vehicles have four. As exhaust comes
into contact with the first 02 sensor, it measures the level of
remaining oxygen. If there's too much oxygen present, the ECU
adjusts to add more fuel to the ratio. If there's no oxygen, the ECU
decreases the amount of fuel in the ratio. The 02 sensor located
after the catalytic converter determines its efficiency and ensures
the first 02 sensor is working properly. After going through the
catalytic converter, the emissions should provide a lower voltage
reading than when they went through the first 02 sensor.
If one or more 02 sensors stops working, you may see a check engine light. A service technician who specializes in 02 sensor repair can assist you.