My R53 has a hard code "O2 sensor stuck lean" and a mil light.
This has appeared after some major mods (listed below) but only after a month of driving some 4000 miles where prior, no issues appeared or existed.
Details: I bought a code reader to check the code(s) because this mil light showed up at about the same time my AFR is displaying an ERROR. It says HTR upon startup but normally goes to 14.7 or so. Instead it pegs at 22.4 and then immediately "Errors out" (Error E8) is displayed and stays errored out while driving.

Vehicle Configuration:
Model year 2002 R53 with brand new engine 4000 miles ago.
During swap:
I installed a Schrick cam.
I added a 16% Pulley on the supercharger.
I also have a 4% overdrive at the crank.
The car has a cold air intake.
It has a stainless header, cat delete, and a Milltek non resonated exhaust, I also added at swap.

I added an LSD while the tranny was out and replaced everything, rad, rad condensor, steering rack, ball joints, wheel bearings, control arm bushings, short shifter, axles, tranny seals, intake, fuel rail, brake lines, added oil cooler, oil catch can, 6 extra gauges (two in center console) basically put 5000 in parts and my labor.

The downstream O2 sensor has the sparkplug trick (a 90 degree tube that pulls the sensor out of the exhaust stream slightly).

I have only been running STOCK injectors; Not good, I know - but I ran out of time.
Regardless, the car ran or runs great, has gobs of power and I have had NO driveability issues (until 4000 mile)
point when suddenly the AFR (says 22.4) and a Lean code P2270 appeared; this worries me. I have put about 500 miles on with the said condition searching for causes. It still runs great and appears to have no issues (no ping, no knock) that I can detect (aside from the obvious - the stated Mil light and AFR).
Note:
I wanted to get bigger injectors and a tune but relocated for work. Now I am settled in and the code showed up. A tuner said to me: "NO tune until you have resolved your lean fault code".

I cannot delete the (hard) code as the tool required is a dealer only visit!
- not going to a Mini shop - so don't suggest that.

Has anyone who has a cat delete and substantial overdrive (boost) on their supercharger ran into this problem (code) stuck lean p2270?

What was your fix?

Some forums say could be a bad O2 sensor, sensor wire, a vacuum leak, or an exhaust leak.

I have thought about swapping the O2 sensor with a simulator but "will that cure my AFR also showing 22.4 and it's (E8 error)?" The AFR gauge supplier feels the error is not a gauge issue as the P2270 exists.
Help is appreciated as always and hopefully others in the same boat can figure this issue out once it happens to them.

Final note:
The reason I drove 4000 miles was a move across country (36 hour drive) needing me to be at a new job on schedule. I would not normally do this (skip injectors and a tune) but my move relocated me and the car (which I drove to my current destination); it got me here performing flawlessly. I began driving around my new town for a month (different altitude) when my AFR errored occurred and a mil light p2270 appeared.

 

Going to move you to Mod section as it appears what you did has more to do with your issues than normal wear and tear.

I am wondering if with stock injectors your fuel input is limited and you are getting too lean. With your mods, I am also thinking you may have more boost than the ECU is capable of addressing.