Montana Griz

I have a 2014 Silverado with 242k on it. I also have been getting the error codes intermittently. They come on for a couple days then go off for a couple months. Definitely have lost some power and gas mileage is down as well. I was going to take it to my local muffler shop and have them replaced, now I'm not so sure. Guess I need to ask the right questions and make sure to get a warranty.