Do a search for " P0240 Toyota Corolla" for better info on that code and the cats. Other problems like poor spark or a worn fuel injector could cause the cats to go bad and trip that code. You need to be sure everything else is working properly before replacing the cats.
Here is a good link to read and try the things that are suggested there first: http://www.fixya.com/cars/t1056719-p0420_code_toyota_corolla
Make sure you have NO exhaust leaks in the exhaust system. You have cleaned or replaced the MAP sensor. You have checked the fuel trim and oxygen sensors and that the ECM is seeing the proper values. Use Denso replacements parts if needed as they are the OEM parts used in these cars.