Preface: I am not a professional!!!! Today
From Total Car Diagnostics for OBDC2 codes the meaning for P0420 is:
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
What does it mean?
. . . It is important for a professional to diagnose properly rather using a hopeful hit and miss part replacement avenue. For instance the catalytic converter may fix the code, but what caused the converter to go out will not be known unless diagnosed properly.