So I have a CEL: P0420 Catalyst System Low Efficiency

I have replaced the follow in an effort to clear code:

Rear O2 Sensor
Cat Converter (nothing special)
Front 02 Sensor
IAT Sensor

I was told a P0420 code can also be cause by misfires so I also replaced the following:
(no misfires are obvious; figured maybe it was unnoticeable)

Plugs (NGK iR)
Boots
Coil Packs
Coil Pack Housing
Fuel Filter


After clearing code, CEL returns after just a few drives (~60 miles)

Any help would be great!

i had this problem in my 97 z. clear code always came back. wife drove it went away. the only thing i could come up with is what the fart can i had welded on the back was not causing any back pressure. causing the exhaust to flow faster and the 02 sensor to not read correctly. reason i think this way is because when i welded on a factory style muffler code went away.

btw try searching in the performance or the maintenance sections.