Post-Cat Oxygen Sensor - Bosch - E46 M3, MZ3, Z4M (11781406622)

E46 M3, MZ3 S54, Z4M bank 2 post-cat O2 sensor (cylinder 4-6, after the catalytic converter). As used most commonly after the catalytic converter on many 2001, 2002, 2003, 2004, 2005 and 2006 model M3 with the S54 engine, as well as the Z3 M and Z4 M. 

This is the oxygen sensor (O2 sensor or Lambda probe) located after the catalytic converter. It reads and checks the status of your S54's exhaust gasses after going through the catalytic converter. The S54 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. Having fresh oxygen sensors helps insure the net cat won't get damaged from an improperly running engine. 

This is a brand new OEM / OE O2 sensor made by Bosch, the inventors of the system and the original supplier to BMW.