Hi, I have been having problems with the catalytic convertor on my girlfriend's '00 GS 4 Door. I keep getting CEL 0420. I tried replacing it with a used cat (presumed to be in working order) but the code came back. I then bought an aftermarket unit and still had trouble. I though it could possibly be the o2 sensor so I bought a new one and switched them around hoping to root out the problem with no luck.

So my question is what's left? Could the aftermarket cat not be working, or is there something else I could check?

I did disconnect the battery and reset the idle memory after each attempt to fix it.

Thanks!