2006 R53 Service Engine Soon light; 2096 lean code?

[m81mclaren]

Hi all, my Mini runs awesome but I had a Service Engine Soon light pop up. I plugged in my Foxwell NT510 scanner and see a code 2096/present fault. "DME oxygen Sensor (bank 1, after catalyst) - exhaust gas composition indicates oxygen sensor fault (bank 1, before catalyst) (mixture too lean).

I am new to this and have been challenged to figure it out. If too lean this is bad as I don't want to destroy and engine and if the computer overcompensates with too much fuel then I'll destroy my CAT right? It did smell quite rich on startups the last year. Car has a resonator delete if that matters and the battery is very weak and I'll be replacing it today.

I cleared the code which worked but after a 10 minutes drive the SES light came back. I purchased a new NTK Oxygen Sensor and replaced the upstream unit given the description. Cleared codes and then the SES light and code came back. Since it is indicating sensor issue I simply swapped the original O2 sensor with the after cat sensor and I still get the same error message.

I dug deeper in the MINI section of my scanner to check the fuel trims and basically the after cat is responding (does not fluctuate much which I think is how it is supposed to work) and the pre-cat O2 is within the range it looks like it should be. So it appears the O2's are working as intended. Where do I go from here?

I searched but did not find any obvious vacuum or exhaust leaks. Short of a inlet and exhaust smoke test by a repair shop is there something else I should do?

Thanks for any guidance and sorry for the long post!

[m81mclaren]