00 plymouth neon cat problem

i have a 00 neon 2.0 automatic
i put a performance header on it and it started givin me the cat code
so i got a high flow cat and im still gettin the code

can anyone tell me what the problem is?

Whats the code?

im not sure i had a guy at a shop scan it and forgot to ask
he said i needed a new cat i told him i already had one and he wanted to charge me alot to figure it out so im hopin theres a better way

Just guessing that you are getting Catalyst Efficiency Below Threshold code. That can also be caused by a bad down stream (after the converter) O2 sensor.

okay ill try changing that out

thanks for the input