A PO420 is a generic fault code indicating a drop in converter efficiency. I realize you replaced the converter and O2 sensors. Did you check to make sure the sensors are switching between a rich.08 and a lean.03 voltage shortly after starting the vehicle. If there is no change (flat line) you have a bad sensor. Most times a bad sensor will set a sensor code and not the converter. However, I have seen situation where the sensors are good enough to prevent the 02 code but bad enough to set a PO420. Also, is the converter new? Were the sensors you used new? Is the converter OBD2 complient?