2000 Honda Civic Exhaust Manifold

Engine Performance problem
2000 Honda Civic 4 cyl Front Wheel Drive Automatic

I have a 2000 Honda Civic Lx, my stock exhaust manifold was craked and so I replaced it with headers, and bought a MagnaFlow OBD 2 Cat to move down to the bottom, to make space for the headers. Although my sensors are connected my check engine light comes on. I have already disconnected the battery for 15 sec and connected back on so that it turns off, but after a couple miles of driving it, the light comes back on. I have also done the testing with the computer and got a P0420 bank 1 code (catalyst system efficiency below threshold) What does the code mean and what can I do to fix the problem. If you can please help me! Thanx.