Downstream O₂ are really only to tell you when the cats are with out I'm not aware of any other function they are not in the feedback loop for performance.

The pre cat are the main sensor dirt feedback for the air fuel mix and if they are reading the wrong info it will definitely affect performance! The engine for example will think it's sending the feedback for exactly 14.6:1 air: fuel butt in reality it's sending 16:1 and you'll get terrible power.

A big incorrect feedback will show up as too rich/lean on the post cat however as the cat won't be able to scrub the poison as it should

Get those new O₂ in and there is a trick on older cars to reset the O₂ heater calibration: run the car With both O₂ precat unplugged for one or two starts ( I would start the car and run for 3-5 minutes twice). You do not need to clear any codes before changing and you should not need to reset the heater calibration unless you get an O₂ heater related error.

Oil on an O₂ sensor lead can really mess them up however; they get their reference O₂ level from the atmosphere through the wiring jacket that's why there's specific notes on not using silicone sealant on the wiring to O₂ sensors. However the post cat are not used for engine performance so that's a different issue.

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