02 lancer problems help please ?

I have a 2002 lancer that makes a wierd whisle sound when I rev it in park it only goes to about 4000 rpm an boggs down some ppl say its my cat converter? It only goes about 20 or 30 now... An just boggs down like it wants to die I took it to get put on the computer an it read up a code p0421 warm cat or some thing that can some one please help the car starts up fine no knocking in the motor at all some one help thanks