Codes P0421 and P0431

This car use only 60,000km. And the engine check light all get on. The fault codes are P0421 and P0431. We suggested the customer use some Fuel Additives, but some days later this happen again. We change the fuel filter, and clean the gas line and inject valve, change the plug. Last time we disassembly the catalyst, check it did not block. The car came back again. The engine light get on again. And the Phenomenon happen to many cayenne. So I really hope to know how to do?