P0420 on a '99 Camaro v6

So I need help figuring out what to do with this code. It just popped up a couple days ago and Im wondering if I should take it to a shop and have them run the tests to see what could be causing it (since there are so many things it could be). Car runs normally and drivability is the same as usual. I have about 120,000 miles on it. Any help I could get would be great. Looks like I will be doing taxes early to get money for this new project! Thank you.

With those miles and age of your vehicle, it's likely the catalytic converter (driver's side) is shot.

Insist on a good quality cat or else you'll be replacing it often.

P0420 Catalyst System Efficiency Below Threshold (Bank 1)

Call the dealer and ask them what is cover under the emissions warranty. If I am not mistaking, GM is 10 years or so. Not sure. The worst case is that they will say no. If the car has an Owner's Manual, take a look at the warranty booklet.