The check engine light will not turn off

I recently installed both knock sensors and harness on my 2004 Silverado
because I was getting a code that indicated that these were defective. But
after replacing them, I still getting the same code. I already rechecked the
wiring all the way to the computer and found everything to be ok. I don't know
what else to do, could it be a timing issue? I did replace the 5.3 with a 6.0 but I
I think the issue was there before I replaced the engine.