The sensor pre-cat is used by the ECU to set the correct fuel/air mix and the sensor after, or at the rear end of the cat is to check the correct functioning of the cat. If it has a reading outside the standards set it will throw an error and normally will light the MIL (Malfunction Indicator Light), otherwise known as the "engine check light). It can be reset with a suitable OBD tool.
Torque Pro for Android being a pretty good example https://play.google.com/store/apps/details?id=org.prowl.torque

Peter

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