I've replaced them. I had a blog of sorts going within my own post on this forum. Basically I was chasing down a few P codes which all pointed to my pre-cat O2s. Replaced both of those and still had an SES light come back. Replaced 1 of my post-cats after some deeper troubleshooting and it's been cured ever since.

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2006 X5 3.0L - Schwarz Black/Truffle Brown
Stock
But I keep it clean