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P0430 in 2000 Maxima.. continuing problems

Have been getting the P0430 error code for over a year. This is inefficient catalyst Bank 2. Finally decided to fix it when emissions inspection was due. Dealer said new catalyst was needed ($1200). had the work done at an independent shop for $700. And they also did the inspection and passed it. The light came back on after 5 miles, the shop said it must have been a bad catalyst, so put a different one on. The light came back on after 40 miles, with the same code. The shop said the Main Catalyst must be bad. so they replaced that for the part cost (almost $200). Light came back on after 140 miles - same code. Shop said the new Main Cat must have been bad… so put a different new one on. Light back on again after 16 miles. Now shop says that it is probably the plugs, which are fouled. He pulled up the Alldata database and showed me a list of likely issues causing P0430 to show that yes it could be the plugs. Through all this he insists it can’t be the O2 sensor, because then it would be a different code. Questions: If I replace the plugs myself, will the code clear itself, or do I have to have him clear it? Have I spent $900 unnecessarily on the Bank 2 Cat and the Main cat, when it was something else?

Is there a temp sensor on the cat? Usually, it is recommended to change the 02 sensor when you change the cat. Plugs are cheap but if they are fouled, there is a cause for that too, rich unburned mixture getting out the exhaust to the cat will cause this kind of problem. It could be the cats were damaged by that, or not, but I dont think they have gotten far enough upstream with this yet.

i am not sure if there is a temp sensor. i am ok with simple mechanical work like changing pklugs etc. can i change the o2 sensor myself? or is it tricky?

Fouled plugs? Rich fuel-air mixture could foul them and help to contaminate the oxygen sensor. You should spend $9 on a can of Sea Foam to clean the fuel injectors. The stuff may help to keep the new plugs clean too. You probably should get a new forward oxygen sensor.