My Nephew has a 2021 Denali he bought used from a GM dealership I have an extremely good rapport with. He had an issue with his truck where the dash lit up like a Christmas tree and had a rough idle. I called those I know in the dealership service department and alerted them that "My Nephew is bringing in his truck with what sounds like the infamous Lifter issue". He brought it in and they first found two faulty Injectors, and still not running quite right, then went for the Lifters. I'm not happy with them going for the Injectors first (which are out of warranty), had a talk with them and they gave him very deep discount. Anyhow, since getting the truck back, he has an intermittent check engine light, truck runs rough, then light goes out and truck is fine, never lasts for a long period of time. He brought it back to the dealership, they have had the truck twice now, put 100+ miles on it and say they cannot replicate, my Nephew drives off and within 20 miles, experiences to problem. I've suggested they check for bad grounds, not sure what they have checked, I just feel they are ignoring the issue. I told my Nephew keep driving till you get a steady light or it blows up....can't fix it if it's not broke. Any ideas WTF is going on ?......does GM have a diagnostic tool that they could hook up and takes a picture of sensor readings 30 seconds before a CEL and during the CEL, to give the tech an idea of what is failing ?