1998 BMW z3 1.9litre code p0420

Question

have a 1998 bmw z3 with a 1.9 litre. last year the catalytic converter lost its internal parts and sent them to the muffler. i replaced the cat with a universal type. the code p0420 comes up and i replaced both o2 sensors. the code still comes up p0420. no other codes come up. what can it be?? is the universal cat not feasible for my car. If pieces from the cat got into the muffler, would that cause the code to show up?? Can use some help with this please. bobby