'03 Silverado P0430 DTC code

I own an 03 Ext Cab Silverado 5.3 litre and the Engine light has come on. I plugged up an OBDII to it. Got the P0430 code. Catalyst below threshhold. I read on another thread that there is an 8yr/80K mile warranty. I bought it used about 6 weeks ago. It has 78K miles or a little over. My son wanted an after market exhaust. Put it on and about two weeks later the light showed up. First question, will that void the warranty replacement? Second, if it doesnt, who is allowed to do warranty work? The exhaust is 3" from the rear of the y-pipe to Super 40 Flowmaster to 3" dual pipe out with 4" tips. Any suggestions will be greatly appreciated.