Car is showing code p0420 Catalyst system Efficiency Below Threshold. What does this mean?

Car is a 2001 Monte SS. 213,xxx miles. It is running good but I got this code and can't get it inspected. I have turned it off several times but it just keeps coming back. I replaced the catalytic converter about 3 years ago with an aftermarket one. The car wouldnt go more than 20miles per hour when the catalytic converter was bad so dont think that is the problem. A few weeks ago I replaced the EGR valve because the car was not accelerating. Anyway now it is showing no symptoms but this code. Should I replace the catalytic converter or what?

 

It's the aftermarket cat con. For some reason they don't like them. Someone here will probably know how to work around it. You may need to have that code deleted from the computer.

 

O2 sensor maybe.

 

y cant u get it expected? is it not a legal cat? anywho check the 02 sensor as rockn said and if that is fine then it is prolly bein caused by ur aftermarket cat

 

Go to AutoZone.
Use their Code Reader and get all the Codes and any pending.
AFTER you read the Codes at the Car ... Go Inside store.

Plug Reader into Printer.
Get Codes and suggested Troubleshooting Procedures to eliminate Codes Printed-out.
Printed out.
Read the Code - Cause - What to troubleshoot - In what order - from the Print-out.

You might find-out that an overlooked maintenance item needs to be checked - a fuse checked or replaced - something silly you haven't thought of.
When my Monte CEL'd ... I got the Print-out that told me to check the O-2 Fuses. I did. One was blown. One coffee-money Fuse saved me from buying a Pair of New O-2 Sensors I didn't need.

The Code is good to know.
What to do about about clearing the code ... even better.
Try it.
You got nothing to lose.

 

Advance Auto pulled the P0420 on mine then insisted it was just the O2 sensor. Lots of research later (and what lead me here) I decided they were wrong, and they were, I had a faulty cat. Everything I found said it will only throw that code it it's the cat, there's another code for the O2 sensors.