Catalytic Converter!

Help, please. I have a 2002 Impala with 92,000 Miles. The Catalytic Converter became completely plugged up at 87,000 mi. and I replaced it with an aftermarket high-flow unit. I also replaced the spark plugs and the car runs like it was new again. Now I get a check engine light and a code set of "Catalytic Converter efficiency below threshold" (P0420).
I have also replaced the B1, S3 O2 sensor. The local dealer insists that the problem is the "Non-GM converter". The muffler shop insists that the system is fine, that the engine must be producing excessive emissions that the converter cannot completely process. Since South Carolina (where I live) does not have vehicle/smog inspections not many mechanics are familiar with the fine points of emission controls and sensors, no one has a an exhaust gas analyzer that I can find, and I have been offered many "maybe" fixes.
Anyone have a similar experience or remedy?

You could try disconnecting the negative terminal from the battery for about 5 minutes. This should reset the computer and maybe it will "recalibrate" to the new parts.
Jim

I don't know if your engine or the aftermarket cat you have is at fault. But if this can help you, I had my first cat replaced at 20,000 miles because it was rotted out and blocked. I now have 51,000 miles and the inside of the second one rattles when it gets hot (normal temperature) and I accelerate from a standstill. Same noise as a loose heat shield but on the inside. I am actually going to the dealer to have the second one replaced tomorrow. My engine was checked twice and passed the emission tests both times. Dealer says it's normal for Impalas to rot out their cats. but that this problem happens to owners who make their cars idle for long periods of time (cabbies and cops). I think that the design is not up to par (just my opinion).

I am trying the battery disconect idea as I write this...
Will post results asap.

Another note, almost all my driving is interstate, 65+ mph, 40 miles to work, 40 more home, little or no in-town use, rarely sit still with car running. I agree that there appears to be a quality issue, I have been trying to find out if upgrades to the engine computer are available and how one can tell if any have been applied to a particular car.

Another note, almost all my driving is interstate, 65+ mph, 40 miles to work, 40 more home, little or no in-town use, rarely sit still with car running. I agree that there appears to be a quality issue, I have been trying to find out if upgrades to the engine computer are available and how one can tell if any have been applied to a particular car.

I've replaced my CAT with a hi- proformance High flow CAT and the car runs good. But my check engine light comes on also. I've found out that the only way to make it go away is to put a O2 simulator on the O2 sensor by the CAT. All it does is tell the car the CAT is ok.

Wierd thing though, my friend has an 01 alero with the same motor. He replaced his cat and his check engine light hasn't come on. I don't understand that.

Where can I get an O2 simulator, or is it as simple as patching a resistor and power into the sensor side of the O2 circuit to send about .5-.7v back to the computer? (my light came back after battery disconect)

I did a search and found a number of sites advertising O2 simulators, from $25-40. Some are plug in and veh specific while others are splice-in and generic. Will probably try one...