so my p0421 code was intermittent now its been on for the past month straight warmup catalyst below threshold

1)there are 2 which one do i replace the precat or the main cat
2)is there any way to check by using measuring blocks or whatever with vag-com to see if my cat is shot
3) im getting a chip soon and i could code out the rear 02s but if my my cats are clogged or something then i would want to fix it (that would be the only reason to replace i dont really care if its just cleaned out and not doing its job) dont want any performance problems from a clogged cat

sorry for the 3 different questions just want to have a couple options if any

any opinions thanks

01 a6 2.7t 122,000 slushbox