2002 Yukon XL 6L won't start or starts rough on

Run a live screen data to it

55 to 62 psi

You answered your own question It needs a starter

Could be the starter motor!

What happens if you bring power directly to the starter solenoid? Will it crank then? You need to try to isolate the problem. If it will crank with direct power, the starter is NOT the issue. Let us know what you find.

P0300 indicates a random or multiple cylinder misfire. With the engine running rough unplug one coil at a time while listening for an RPM change or for the engine to run even worse. If there is no change from one or more coils than check those coils for spark. If there is no spark out one of the coils swap it with another coil. if there is still no spark replace the coil.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

What are the symptoms ?
Cranks but does not start ?
Cranks for a long time then starts ?
Does not crank without a jump from another battery ?

You need spark at the right time, fuel pressure at the injectors, and normal compression.
Have the spark plugs been changed in the past couple of years ?
Have you checked the fuel pressure to see if it is enough ?
Have you sprayed carb cleaner into the throttle body to try to make the engine try to start ?
Does it sound like it has normal compression when cranking ?

Define - It attempts to turn over but won't catch and run ???? When you turn the key to start does the starter engage an spin the engine ? This would be a crank no start condition . If the starter doesn't engage an spin the engine this would be a no crank no start condition . Sorry to ask but i have only been an auto tech for thirty years an have heard a lot of different discriptions for these simple actions . crank no start or no crank no start . Very different diagnostic procedures . So when you turn the key to start , does the starter engage an spin the engine ,but won't start an run ?

you need to do a injector clean. If that doesn't fix it, take it to a auto parts store that can check for codes.