Post-Cat Oxygen Sensor, Bosch - 550i/650i/750i, X5/X6 50i, Z4 3.0i

Oxygen sensor after the catalytic converter on F10 550i, F06/F12/F13 650i, F01/F02 750i, E70/F15 X5 50i, E71/F16 X6 50i, E85 Z4 3.0i/3.0si, and E89 Z4 sDrive30i.

This the oxygen sensor (O2, monitor, or Lambda probe) located after the catalytic converter. It reads and checks the status of the exhaust after going through the catalytic converter. The engine computer uses this data to verify the cats are working properly. A failed O2 sensor can set several different types of fault codes so it's a good place to start if you have O2 sensor or heater circuit faults. Most cat manufacturers will insist that you have new post-cat O2 sensors when you install new cats. This is a new OEM O2 sensor from Bosch, the inventors of the system and the original supplier to BMW.

Z4 models need 1 post-cat sensor.
550i/650i/750i/X5/X6 models use 2 post-cat sensors (1 for each bank). The part number is the same for each bank.