[davidcle]

My car is 2013 535i xDrive, and the engine light is up with the error code P0420 "Catalyst System Efficiency Below Threshold Bank 1"
I already replaced both pre-cat and post-cat O2 sensors and 6 new sparkplugs. Then I cleared the code, but it came back after my test drives.
Is my cat really bad and need to be replaced or is there any other reasons that caused this code P0420?
Will the leak at the connection of the cat to the exhaust pipe cause this problem?
How to check if that connection is leak?
I'm not sure what to do now. I try to check other possible causes before deciding to replace the cat, which is really expensive.
I also attached some pictures of the signals I capture by my scanner for your reference.
I also wonder if oxygen sensor spacer that I heard will help in this case?
Do you have idea or suggestions. Thanks very much.
--david