how can i tell location of lambda sensor b1 s1 ?

hello i have a audi a8 2.8 30v and done a diagnostic check to descover lambda sensor b1 s1 bank 1 is faulty i have replaced it and changed it over (there are 2) but still get the fault , is there maybee 3 sensors on this
car ? and does anyone know which one is bank 1 b1 s1 ? left or right , thankyou
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