I'm getting a P0455 P on my scanner

Hello and welcome to 2CarPros! Generally speaking, for an EVAP system leak diagnostic, the smoke test would be the better way to go. I've done some digging and found a wealth of information on that code for that truck. GM has put out a service bulletin, suggesting the possibility of a leak in the fuel tank or filler neck. An easy way to check this would be to simply fill it up until it's coming out of the gas door (I know, we're not supposed to do that, but once won't kill it). Then you just get under the truck and look for the leak. Simple as that. Prodemand (a mechanic's subscription database service) put together a graph of the code and common fixes, suggesting the gasa cap to be the most common cause of this code, and then a few other things in succession. That graph is in the images that I've uploaded. Finally, GM has put out an official diagnostic process for this code. While you probably don't have the scanner necessary to do some of what it suggests, you can still follow the logic and use it to guide you through a fairly long and drawn-out diagnostic process.

Go ahead and study the information, perform whatever testing that you actually can, and report back to this thread with the information. I know it's a bit of an information dump, but try and pick through it and do what you can. Good luck!

Harry

Just depends on where it is. The actual neck is part of the tank. If the leak is there, then you have to change the whole tank. You might as well change the hose too, being that it's rubber and probably getting dry rotted anyways. If it's just in the hose, then only change the hose. No need to change the tank in this case. See my logic here?