After a quick search, I believe your O2 sensor (also known outside the US as a Lambda sensor, like Storamin said) will be part #11781734393. But verify this at
www.realoem.com. Just enter your vehicle information, and you can look at all the parts in detail. To find your vehicle's production date, look at the sticker on the driver's side door; there should be a month/year of manufacture.
You can also read the OBD fault codes to narrow down the problem. To do this, go to
http://www.pelicanparts.com/BMW/tech...de_Reading.htm
Hope this helps, and let us know how things go
