Hi everyone...got a code from my 98 civic.P0420 Catalyst System Efficiency Below Threshold.what could it be??i think i had this code(not 100% if is P420 but im sure i had a code) 6 months ago and i changed the lower o2 sensor with a denso brand was told thats the oem but anyways,now is back again...anyone had this same code before and got it fix??hope is not my cat cause u know how much that would cost ,could my throttle effect this? cause i have not clean it since new.and yes i'll will clean that asap

 

You already changed secondary O2 sensors, so chances are very likely your cat is toast.

 

Forgot the mention,i just got my emission test (don't know what u can it in the states,but im sure u get the idea) done two weeks ago and it pass with very low readings.just don't make much sense to me u know.