Help! I'm losing my cool over this. My 2002 3800 Y87 Camaro with 107K miles on it continues to throw a "P0420 - Catalyst System Efficiency Below Threshold (Bank 1)" code no matter what I've tried to fix it.

Here is what I've done so far:
1. Replaced catalytic converter
2. Replaced all 3 O2 sensors (2 at manifold and 1 post catalytic converter)
3. Replaced Engine Coolant Temperature sensor. (thought that false hot reading was causing incorrect air/fuel mix)
4. Sea Foam treatment to clean injector nozzles.

I'm about out of ideas. Has anyone else seen this issue? Any other recommendations?

 

Have you cleared the code after each of those repairs? It doesn't go away on its own, even if the problem is fixed.

Mine (an LS1) is currently throwing the same code -- I am having the converter replaced under warranty. Hopefully I don't end up having the same problem...

 

Yes, I have cleared the code after fix. After a day or so it comes back.

 

This is from my 1996 factory manual, but should apply to your V6 as well. Right click and view image to view at full size.

 

mine has been throwing the same code since last year when i put and aftermarket magnaflow exhaust with an aftermarket hiflo magnaflow cat on it. i dont know what to do to make it go away either...ive heard that an 02 simulator is needed. dunno where to get one, which one to get, and where to put it on the car...help please

 

I think your problem has got to be the cat. Look above.....the manual states to make sure that the cat is OEM spec. Your highflow cat is not working to the PCM's satisfaction. I know for certain that a muffler will not hurt anything. I ran one on my 96 3.8 without issue(dynomax superturbo from RKsport).