2004 GMC Sierra 1500 4x4, 122k miles, automatic transmission, 5.3L V8 vortec engine
code P0455 (EVAP system leak)
Been having a lot of trouble with this one. I've taken it to mechanics several times and they either can't figure it out so they want to throw expensive parts at it, or they make a repair and the check engine light seems to only briefly go away. I can't afford to keep throwing money at this but I want to fix it. I've been doing most repairs myself lately to save money. Does anyone have any advice, or has anyone had this problem in their Sierra/Silverado before and what was the issue? After doing a little bit of research I've heard this code is often accompanied by other codes, but my truck is only giving that one code, nothing else.
P0455 is large leak. I suspect a slit/ cracked rubber hose at charcoal canister on top side - hard to see. Smoke machine test will find that. I made one from YouTube video, which can be used every year.
You need a good mechanic to use smoke machine to find leak(s).
I had two different mechanics do just that, they couldn't find any leaks anywhere.
Find one that knows what he is doing.
Yeah, that may be my only option, I was hoping to find someone who had experienced this before and could give me some advice because I wanted to fix it myself, money is pretty tight. I also live in the Mojave desert and there aren't many options for mechanics unless you want to drive hours and hours away.
The easiest thing to try is a new gas cap.
Scotty has several videos about diagnosing EVAP problems. All you have to do is search his Youtube channel.
https://www.youtube.com/c/ScottyKilmermechanic/search?query=evap
I probably should've addressed this in my post, but I did that. Went through several gas caps from autozone and oreilly's, but the check engine light/code persisted so I assumed the gas cap was still at fault. Ended up buying a brand new one that's exact OEM from a chevy dealership and it still hasn't fixed the problem. This has been an ongoing thing for quite a while now so I apologize that I didn't include this information already.
You need to include everything that was already tried in your questions please. Including what the mechanics did. Thanks.
I will take a look at his videos though, thank you.