2012 MERCEDES-BENZ S550

I purchased this vehicle in July 2020. I did a through check for recalls, warranty problems and could not identify any with this vehicle. From the get go the check engine light would come on. I checked the codes and they indicated an "electrical fault on bank 1, oxygen sensor 1. I replaced the wiring harness on that sensor as well as the sensor. From then until now the car would code weekly about the same issue. Eventually I started using a catalytic converter cleaner (it cleans off sulfur deposits) in an attempt to solve the issue. It would work for a couple of tanks of gas and then continuing coding. I asked MB on numerous occasions about what the issue was. They all said they had no idea. It looks to me like they knew what the issue was, but kept it to themselves. Several weeks ago I received a letter in the mail(not dated by postmark) but the letter was dated August, 2023. The letter outlined an issue with the catalytic converter system identifying my vehicle by vin. The same problem I have had all along. The letter came at a time when the car codes almost daily with the same issue as always, ONLY NOW THE VEHICLE STALLS occasionally while waiting at stop lights. I said great. I called to get an appointment to fix the issue only to be told that the fix window had passed. I told them that I just got the letter. They told me that the fix expired in December 2023. I called MB hotline at 1-800-367-6372 to see what was up. They said they would get back to me in 3 days. It's been a week so I called again and they advised me they can't help. It's an issue for the 2011, 2012, 2013, and 2014 MB S550 W221. Why is this not a recall and why won't they fix it. It's an expensive repair. They sent me the letter and said it was expired 6 months before I received the letter. Can you help?