Why is my check engine light still on

You should have had the battery NEGETIVE cable unhooked during knock sensor replacement on the 5.3L engine so that would have cleared the codes .. Now IF for 'some reason' the battery was not disconnected, you can try unhooking the battery NEGETIVE cable for 10 minutes to see if codes clear and light turns off... IF that doesn't work - then a problem still exist and more testing is required!