P0420 code uphill2 02 sensor catalytic converter

What is your question?

That code came out when they put scanner on my 08 malibu 2.4L want to know what to do cause the car loses power ? And when you put the car in drive and you step on the brake the car starts to vibrate a lot. When you take off from a parking place you can have the gas pedal all the way down and the car feels like it can't take off. Should I replace the 02 sensor or what would you recommended me to do?