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P0420 code on 05 Nissan Maxima

Keep getting the P0420 code on this vehicle. Have changed air filter, spark plugs, run fuel injector cleaner through the tank on Shell 93 octane fuel. Drove 430 highway miles to clean out the carbon and the SES light is still on and when we check the code it says Catalyst system effiency below threshold bank 1. Please help! How do i get this fixed?

This code is often set by oxygen sensors that are nearing the end of their useful lives. I suggest replacing the O2 sensors. They are much less expensive that a catalytic converter.