I have 2001 Accord DX with 86k miles. CEL wenthe code was p0420. Catalytic cnverter was making a lot of noise becous it breakdown inside. I replaced CAT with bolt on OEM replacement, cleared codes and I thought its going to be ok. But after few days CEL comed on again with the same code.

What should i do now?

 

Have you checked the rear oxygen sensor? OBD doesn't have any other way of knowing if the cat is working or not other than the rear o2 sensor, so it could be damaged or fouled.

 

return your aftermarket cat. and get a Honda cat. as the aftermarket replacement cat. do not clean the air enough to keep the light out of course the Honda cat. cost way more but you get what you pay for.... you can always do the fouler trick.

 

whats the description on this code? is it cat efficiency below threshold or,......

 

yes, cat efficiency below threshold

Honda cat cost almost 800$ so, NO THANKS

 

if it's a ULEV OEM is the only cat. that will keep the light out... without the fouler trick or o2 simulater. or removeing the bulb!

 

u need to do a swing test and observe the second o2. for this, i recommend using a scanner.

rear o2 should not be as responsive to throttle as the primary o2. if both 02 (pre and post cat) are swinging at almost the same rate, this issues a code which you have right now. which inturn means that your cat is not working anymore.
as for changing a cat, i do recommend getting oem ones specially if you are planning on keeping the car longer.
theres a reason why oem cats are expensive than what a regular muffler shops offer.

 

take your 02 sensor off and clean it. I bet its covered with carbon. If the o2 sensor has turned white. its gone foul!