I have a 4.3 v6 sonoma w/P0420 DTC, replaced the Cat converter, gas flter, muffler,chkd exhst pipe leaks; lite still on?

Question

The catalytic converter installed is a universal type Manufactured by CartexLa. A mechanic has told me the reason the light does not go off is because the Converter installed is a universal
and not the exact part called for the specific vehicle. Is this correct. I have had the vehicle diagnosed by four places and three of them come up with a P0420 code and one came up with
a 0171 code. If the check engine light does not go out and I still get a single code of P420, can
it be the downstream O2 sensor or any other sensor. Incidentally, the catalytic converter and gas filter were changed about 11 months ago. The muffler, patched exhaust pipe welds and
spark plugs were changed this Jan 21,2012. What else can be wrong?