Not sure what your asking about how bad it could screw them up, but to answer your setting a code question, no they dont always have to set a code. My truck all the sudden had a rough idle then around 50 mph would shake like crazy and since I used to work for as a tech for an international dealership (03-06) I had worked on several vehicles with the 6.0 (which is why I bought one
)so I knew what a bad injector felt like. I took it in under warranty and 3 injectors later, which also no code after each one went bad im up and running. They do a cylinder contribution test or cutout test.