can someone please help me?!! I've taken my car to the mechanic over and over. I've had the cat converter replaced on passenger side, the MAF replaced, all spark plug wires, oil filter, spark plugs, and oxygen sensor. And still my check engine light turns back on.

 

P0420 = cat on drivers side (LHS).

Find a better mechanic/shop.

 

Yep, passenger side is P0430. Possibility you might still be under the GM extended cat warranty (10yrs/120K) depending on mileage and when the car was placed in service.