i have an 00 si w/ b16a2
cel p0420=catalyst sys. efficiency below threshold bank 1
from what i found searching,cat sys eff below threshold should be secondary o2 sensor or poss bad cat.(please correct me if i'm wrong)
but i thought bank 1= primary 02 sensor
so which o2 sensor should i replace first because i'm pretty sure its not the cat
any help would be appreciated, because of course this happens right at emissions time

 

The secondary sensor is the one that checks cat eff... More than likely its the cat not the o2, if the car has less than 80k miles it may be eligable for warranty

 

thanx
no warranty and car has 91k on it
dc headers and tsubo exh

 

just checked out the cat and it seems that there is enough pressure coming from tailpipe and it doesn't seem to be down on power
don't hear any rattling from it either
so i guess i should replace the o2 sensor first