I have this same problem, with the same 0455 code as well.. I noticed when I remote started my car this morning for work, it just kept turning over... It eventually started, and like everyday I park on an incline ( for me its the front of the car Higher than the back ) it runs fine after the initial cold start..
For me the last thing Ill buy is a gas cap as it has not fixed the issue on either of my prior 3800s..
As I see it there is only a few things led to that code
- Vapor canister purge valve ( GM-ACDelco p/n - 2141105 )
- Vapor canister vent solenoid ( assuming this is the purge solonoid )
- EGR
- Vacuum leak
Now the vapor canister vent solenoid , I believe is what goes bad, the one under the rear drivers fenderwell
