O2 sensors

[POBeemer]

How do aftermarket code readers discriminate over which O2 sensor is at fault?? I had my 98 740I scanned today and it said something like, bank 1....... O2 sensor slow response. I know my vehicle has 4 sensors two are in front of the cats and 2 are after the cats but I don't know how the code readers identify which one is at fault specifically. Help if you can.

If this sounds to confusing, i'm just asking if each sensor has it's own name when you read the code reader or are you stuck replacing all 4 when one is bad... I would think the code reader could specify.

Thanks......

Scott, pleased as punch with my E38