What does a ******** Catalyst(TWC) system, low efficency service code mean? and where is it located

DTC P0420: Catalyst System Low Efficiency Bank 1
Circuit/System Description
Important: A new converter with less than 100 miles on it may set DTC P0420 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 hour may correct the condition.
A 3-way catalytic converter (TWC) controls emissions by converting hydrocarbons (HC) and carbon monoxide (CO) into carbon dioxide (CO2), and oxides of nitrogen (NOx) into nitrogen. The TWC also stores oxygen. When the listed idle conditions follow the listed driving conditions, the Catalyst System Efficiency test will run. The powertrain control module (PCM) monitors this oxygen storage capacity by comparing the pre and post-catalyst oxygen sensor signals while adding and subtracting fuel during specific idle conditions. The PCM compares the pre and post-catalyst-oxygen sensor signals to determine if the oxygen storage capacity of the catalyst is degraded.

https://www.youtube.com/watch?v=CflIvd5VOoA&t=293s

Any PO420 or PO430 Codes---means your exhaust converter is

below efficency,time to replace,worn out,won't pass inspection.

I think you got misinformed about what fault P0420 is, the GM manual says this: "P0420 - Catalyst System Low Efficiency" What does that mean? it means the main catalytic converter in the exhaust system is worn out, it could be the Catalyst monitor that is bad, it is what triggers the code when it reads a certain voltage from the system, but that is rare, you could replace it (the catalyst monitor) and see if the code clears, if not you will need a new main Catalytic Converter.

CODE PO102 IS MAF SENSOR CIRCUIT LOW FREQUENCY CONDITIONS / CODE PO420 IS CATALYST SYSTEM LOW EFFICIENCY BANK 1 CONDITIONS. BAD MASS AIR FLOW SENSOR WILL AFFECT CONTAMINATE DAMAGE CATALYTIC CONVERTER DUE TO RICH EXHAUST EMISSIONS.

For the lean condition I wound look for vacuum leaks.I you don`t find any then replace the o2 sensor for bank 1.As for the po420 if the exhaust system is not bent or damaged you will have to replace the catalytic converter.The catalytic converter has a longer warranty then the regular vehicle warranty itself so call the dealership and find out if the converter is still under warranty.The usual warranty I believe is around 80k.mileage years is 7 or 8 not positive.Hope this helps.Good luck.

please go to the provide link for diagnosis of this code,but the bottom line here is the catalytic converter may be worn out

http://www.p0420.com/

Catalyst System Efficiency Below Threshold Bank 1
this is code P0420.
It's telling you converter is not as efficient as it should be.You can drive it like this forever,or spend to replace a converter.

sound like a injector problem

Vehicle needs a Primary or first position Oxygen sensor, bank one just means it's the O2 sensor before catalytic converter

Replace the catalytic converter.