I had that same code but a p0300 also and was started to blow white smoke out of the exhaust. I ended up just paying out of pocket for injectors and fuel lines. It wasn’t worth the fight with GM. My mechanics checked everything else out also. Spark plugs were fouled and the cylinders weren’t washed out with fuel. He changed the oil though and did an upper induction cleaning I just got the call today that it’s done, a month and a half later, about to write an $1800 check for 8 new injectors, new fuel lines, oil change, and induction cleaning. If you do just the injectors and fuel lines it should be around 1,200-1,400