Upstream sensor is before the cat will have no connection to monitoring wear of the cat. I don't remember my n55 pre cat sensors read but you can find a YouTube video where somebody shows a graph of pre/post sensors for your exact engine. Look for different chassis if can't find in X5.

I've had very little luck with non oem O₂ sensors (rounds to zero), but I've installed $20 Amazon sensors that worked for months.

If further diagonals leads to pre cat are shot get the highest rated knock off with a 12mo warranty if you sell the car jus tell the buyer the O₂ were replaced and have a year warranty you'll give him the receipt for exchange.

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