96 Camry V6 OBD code PO420 cat converter 6 mos. old

Well maintained, single owner, 96 camry v6 with 171K miles. Due to rust (Massachusetts), replaced front part of emissions system which includes the catalytic converter in Jan. 09. About 3 or 4K miles later, a week or so after replacing air conditioner compressor, in June, CEL came on. We’ve tried resetting the computer, has come on 2 more times.

The reading is PO420: BBCatalyst system efficiency below threshold - bank 1.

After researching I’ve found people who never seemed to resolve this by replacing the cat or 02 sensors.

I have two questions:

In MA, for inspection, are emissions evaluated by the OBDII sensor or by actually measuring the tailpipe emissions?

Is there a way to determine if it is a bad cat or one or both of the O2 sensors that is the problem other than keep trying the options?

I’d love some advice on the route to take to try and resolve this. The car has been great. Even if we replace it, we’d rather give/sell it than trade it in, if we can be sure that we’ve fixed the problem.

This site http://www.p0420.com/ has some good, useful, information on the P0420 problem; but, I think it’s a little too quick to recommend changing the catalytic converter.
The health of the oxygen sensors and catalytic converter is very much dependent on the health of the engine. Burning oil, antifreeze, silicon, or too much fuel, poison the catalytic converter and the oxygen sensors.
Put the engine into tip-top health, and the catalytic converter may recover. Check the output signals of the oxygen sensors, and the other sensors before just changing parts. [As an aside, too many mechanics do just change parts without checking.]

Thanks. I’ve read all of the articles posted there, and they are helpful. At least I can check with the mechanic as to whether he measures the O2 sensors, the cat, etc., or is just taking a guess.

The PO420 code didn’t come on before I replaced the cat converter. There was noise in the engine area and one of the front pipes of the emission system was worn, couldn’t be welded, so the piece needed to be replaced, which included the cat converter. It was 6 mos. after it was replaced that the code started showing up.