P0420 after changing parts

Hello guys. First of all let me start by saying that I watch Eric’s videos for about 10-11 years now, big fan. It is my first post here. I really really need your help with my car.
I have a 2011 Suzuki Swift 1.2 94hp with 256k km on board.
Recently, I got the p0420 error code. Started initially by replacing the downstream o2 sensor as it was the original one and the values were all over the place for it. No changes.
By now, I have replaced the following: Both o2 sensors with OEM ones. MAF and MAP sensor with OEM parts. Catalytic converter, not oem but OEM quality certified (whatever that means). Also, replaced all the exhaust gaskets and checked the pipes for leaks, all good there.
After all this, I still have the check engine on and the ERROR code p0420 active. I don’t honestly know what I can do next. I don’t really want to use the o2 sensor extension hack with the spark plug non fouler thing because I have all brand new parts so it should be fine.
The car works much better than with the old parts, no more rough idle, no more smudges on acceleration and deceleration, fuel consumption went down, but the error code still present. Here are 2 video with live data from the car, one is at idle and one at around 2k rpm. If you have any suggestions, please let me know, I have no more idea on what to do next to get rid of the stupid error.

Topic

[Adrian IlincaParticipant]

earlier today, noticed something strange. With the AC on, the o2 downstream sensor started to randomly switch from 0.7 to 0.1, 0.2. When turning the AC off, it stays steady around 0.7. I start to think that the car is possesed by some o2 sensor demon. Tomorrow I am going to clean all the grounds I can find in the engine bay, maybe there are some corroded or something and giving strange signals in the electrical system. While this switching appears, the engine runs perfectly fine, no hesitation, no rough idle, no nothing.

[Adrian IlincaParticipant]

Nobody?

[Author]

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