Weird P0420 code in 01 Cougar

Hi guys this is my 1st post so here goes..I have a 2001 Cougar auto transmission. I have a check engine light for P0420. I have no exhaust leaks, flowmaster muffler, cold air intake. I have replaced fuel filter, plugs, and wires. I dont believe that the cat is the issue unless its because its a universal.

Heres the weird part... I found that if i create an exhaust leak before the main cat, my cel goes away. But if i patch the leak the cel comes on. Any ideas of what to do? I really appreciate your help and look forward to contributing to the forum