buy an O2 sensor bung and install the bung really near to the original spot and put in the CARB certified cat as is.

moving the O2 sensor a few inches forwards wont hurt.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)