catalytic changed and engine light comes on. code po402, what does this mean? on 2003 Toyota Corolla

different mechanics give me different answers for a changed catalytic converter. had it dx. and engine light still comes on. The last mechanic stated that i needed a double headed converter and I checked auto part store and it stated that I only needed a single headed converter and not a double headed converter? can you clarify this, please?

Asked by for the 2003 Toyota Corolla
P0402 is not the converter , it;s EGR system. P0420 is for converter below efficiency.