You may or may not get a check engine light. You will however get P0401 and P0404 if you pull the cat. Sometimes they trip the light, sometimes they dont. If so, be prepared to get a fingers stick, or you could go to your local dealership and get the latest reflash as I hear it has also taken care of all egr codes.
Before I put a full exhaust I would get the new dealer download, It will be harder to do it after. Just tell him you got these two codes a few times already and had them reset. There is a flash just to fix it. My truck is proof it does fix it without a finger stick. I got the 401 and 404 just from a cat back 4" duals. had the new flash and it's gone. Then again mine is cat back only..
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