OK, just a little update. Ordered the upstream O2 (air/fuel ratio) sensors and received them earlier this week. Trying to get them in this weekend.

I have a nephew-in-law who works as a tech at a Toyota dealership. Regarding the vacuum leak theory, he is doubting it b/c he thinks if that were the case it would be affecting both banks, not just Bank 2. I see his point, but then I also feel like it would depend on a lot of factors, not the least of which would be the location of the leak -- if there is one.

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2009 X5 3.0
2001 X5 4.4 - Sold