I got this code p0420 code catalyst system efficiency below threshold, I did search the solution online, change the o2 sensor upstream and downstream, change the coolant temp sensor sparkplug the code is still coming back help please thanks a lot.

 

your catylitic convertor is bad

 

sorry forget to mention that i already change the catalytic converter, i replace with aftermarket cat highflow, but the code is still coming back.

 

help please!!

 

i had the same code and i replaced the catalytic converter and the code went away

 

year make and model would help.but it might be on becuse of the high flow cat

 

The high flow aftermarket cat isnt as efficent as the oem unit. So the computer is still thinking that the oem cat isnt working like it should. Look into an o2 simulator or just live with it

 

1998 honda civic ex. its suck to see a check engine light, i guess im just gonna leave it alone, thanks guys