You will get lots an lots of POST cat errors if the cats start going bad. You can extend the cat life a year or two by adding an extension on the o2 bung hole. You can watch the live data of the pre and post cat to confirm aging cat. My wife's x5 has old cats I put extension on about 10 months ago and we will get O2 "inefficient cat" errors occasionally like every 6-8 weeks but when I monitor real-time data when engine (cats) fully warmed up, the curves show cats are still working just not 100%

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)