How do i determine which O2 sensor is bad? The downstream or the upstream?

What is your question?

I got a check engine light and i got it check and i have the error code P2096 post catalyst fuel trim system too lean bank 1. -Definition: Code P2096 indicates the PCM determined that the difference between downstream fuel trim and the established "lean" threshold value exceeded the maximum allowable limit for a specified number of samples during the test period. As far as I understand this is a downstream O2 sensor problem but i am not sure. If anyone know this better please help.thanks

What makes this problem better or worse?

Not sure yet but i felt the car almost died during idle few hours before the check engine light came on

How long has your 2015 BMW 228i had this problem?

1 day ago