If a cat is going bad or bad, P0420 or P0430 codes are present. Assuming O2 rear sensors are working properly.
Rear V reading should be around 0.45. The code may have been set because the rear sensor took too long to warm up. I would reset it and drive it. See how long it takes to come back if it does.

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'06 X5 3.0i - bought @143,123 miles (12/26/20)