Your cat is bad. go get it fix at dealership, depends on your 2004's first "sale" date, it might be covered under 8 yr 80K emission warranty
if your warranty is out, then u might start looking to go either Catless (test pipe), another used cat, a new stock cat, or an aftermarket cat. Aftermarket cat is probably the cheapest
Feel free to ask every single little thing on this forum.
a lot of cool dudes will answer your questions accordingly