sikoop74

my car is a 00 si w/ catback exhaust, 4-1 header, and cai. it was making hissing noises during acceleration. turned out to be a severely clogged cat so i replaced it w/ a hi flow. now MIL lites up after about 50mi w/ P0420 catalyst efficiency below threshold. replaced primary and secondary o2s but the code just keeps coming back. inspection is coming soon so any input would be great help. all scheduled maint performed (ie timing belt, valve clearances, etc.). a new hi flow cat and o2s probably won't pass...it was still reading hi HCs @ idle w/ a 5 gas analyzer.

sleepN

How did you clogged up your 2000 Si cat already. I think the new cat is not doing its job of cleaning the gases and the 2nd o2 is sensing to high of HC's. I'm not sure but you should figure out how your old cat got clogged first cause you're gonna have another clogged one again.