If you had a vacuum leak, it would have shown up as system too lean (too much air in the system). When it is too rich, either your system is dumping too much fuel, or the system is not reading the mixture right. And yeah, an O2 sensor can go bad at any time, but it can also last over 100k miles too. The way you can tell which one it is, the system will tell you, upstream or down stream ( upstream-before cats, down stream-after cats) and then bank 1 or bank 2 ( bank 1- where the number 1 cylinder is, bank 2- the opposite side). Hope that this will help. Let me know if you have more questions.
'09 E92 ///M3 Interlagos Blue, 6MT
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