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the sensor before the pre-cat is monitoring combustion so as to control engine fueling. the sensor after the pre-cat is monitoring whether the pre-cat is working. there is no sensor for the main cat.
the pre-cat, and secondary air injection system on the engine, are north America specific systems due to higher start-up emissions standards. you typically don't find these systems on 'rest of world' cars. that's why one of the tricks if putting catless headers on your car is to get a 'row' flash, because the computer no longer monitors whether the pre-cat is working or not.
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