E320 97 p0422 and P0455

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Hi All
If you can help me on my issue. I have changed the o2 sensor recently and the car passed the emission test. however,after about a week P0455 came on and I changed the Gas Cap and cleared the code, and then the code came back again (P0455). Then after another week P0422 came on with
P0133 (o2 sensor again). Any suggestions? should I start with the P0455 or Do you know if they are related? I have some friends advising I should change the charcol canister Is that a good start ? and If this is going to solve P0455 will it solve the P0422 or should I start with the O2 diag prob (sensor 2) and then the cat convertor ? please advise !!!