Can anyone tell me what’s wrong with my Nissan 350Z 2008?

The problem started last year when my car had only 13000 miles. After the Engine malfunction indicator came on I took my car to the Nissan mechanic and had it checked . The error codes indicated that the problem was with the A/F ratio sensor bank 2. The error codes were cleared with consult 111. The mechanic told me that it could have been a problem of low octane fuel.
After 300 miles the light came on again. The process was repeated again and error codes cleared. After I had this problem for 3 or 4 times the mechanic changed the A/F ratio sensor with a new one from Nissan. In the meantime I also changed the battery.

Although the engine works fine the light came on again after 400 miles.

The car currently has 19000 miles and is still in brand new condition.

 

I would check for an exhaust leak. I had a slight crack in my cats and it threw P0420 for the longest time.

 

Lemon law