a bad o2 will lead to sputtering and an uneasy, lurching idle, especially when cold. My car is in right now with a bad lambda o2, it didnt seem to lose much power though, and no limp modes etc. Mine is hopefullt going to get taken care of under cpo, but if not the dealer will rob you blind on them, 800ish bucks to replace one. Tischer sells them for around 180 i believe, and installation isnt very difficult from a mechanical standpoint.
E88 N54 Alpinweiss/Coral Red/Motiv HTA 3586r Tial .82AR/Other stuff...652WHP
F30 N55 XDrive EBII