1999 Es 300 Code P0420

[bronco2454]

Car is a 1999 ES 300. Car was throwing codes for p1354 and p0420. Replaced OCV and car is no longer giving p1354. After driving about 60 miles the P0420 code came back. From research on this site it appears that one of the O2 sensors is the likely culprit. Do you really have to remove the drivers seat to replaced the after cat sensor? Car is CA emissions. I do not live in CA, if it is the cat does it matter if cat is not approved for CA. By this I mean is the programming in the computer different and cause problems?

My mechanic cleaned MAF when the car had multiple misfire codes and now this P0420 code is coming up. Could it be related?

Any thoughts would be appreciated