My OCD got the better of me not long after I got my 3.0i. So I replaced the original front O2 sensors at around 170k miles, hoping for better MPG. No codes or engine issues before the change ...

And afterwards, no change at all in MPG, idling, accelerating, even imagined improvements. Nothing! I was hoping for at least some placebo effect on this one.

So I'd vote to leave them in there until you get a code or some measurements that suggest there is a problem.

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2001 X5 3.0i, 203k miles, AT, owned since 2014