I have a 2002 impala it showing troubleshooting p0420 OEM brand: domestic. What is the major problem

Could be intake gaskts leaking causing this or could be bad cat. convertor on bank 1 cylinder head, could be as simple as a bad spark plug causing that bank to run nonefficient. Bank 1 is the cylinder head that is the closest to you when standing on r side of car looking at engine (passenger side) you will see one head which is closer to you than the other...that is Bank 1

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