2000 Infiniti I30 P0430 Bank 2 code

The 'service engine soon' orange light appeared and stayed on 2 months after I had replaced defected Bank 1 rear O2 sensor. The diagnostic code indicated
P0430 B2 problem, which meant bad catalyst(or catalytic converter per one mechanic), I smelled plastic or rubber burning order when that occurred first time, but the smell soon disappered and never came back. The mechanic cleared the code, the light came back after I drove another 1000 miles. The diagnostic code showed P0430 B2 again. I went to a 20 years experienced Muffler shop, the
techniciian said he knew which coverter is bad, and
replaced the last converter the one just before the
muffler, with a new after market unit. He disconnected the battery to wipe out the orange light. And I have driven about 70 miles so far, the
light has not come back. According to the code
P0430 B2, the guy in dealer's part department said
it should be the first converter right off the engine
exhaust manifold at the front of engine compartment. Can I get some advice about this puzzling problem?