P2096 code

  mercedes/c-class/2001-2007

My MB c240 has thr trouble code 2096 when i check the live data i see all the fuel trim long term and short term in both bank are negative they reach -9./. Which mean the system is rich but the code 2096 give me post cat bank1 is lean why this contradiction ?
I hear all time a siffering sound from the engine

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