OBD P0455 trouble code
 

Has anyone had experience with this trouble code that indicates a "large leak in the evaporative emissions system". I'll try to locate a charcoal canister and hoses, but was wondering where a sensor would be that sends signal to the computer? It seems coincidental that I first got this trouble code the day after an emissions test where I think they improperly removed the gas cap to test it while the car was running. I replaced the gas cap with a new one and disconnected the battery to reset the code, but it came back within a few weeks. Thanks for any help!

There is a evap tester in the Motronic system. You need a Porsche tester to locate and test any faults. Replacing the canister without diagnosing the fault doesn't gaurantee a cure,

It seems that there must really be a pressure (or vacuum) leak in the fuel tank. When I remove the gas cap for re-fueling, there is no longer the loud sound of air flowing into the tank. I've replaced the gas cap and both new and old tested OK, but have same results. Someone has told me that the problem may be in a check valve in the fuel filler or attached to the filler, that can be accessed by removing the inner fender. Does anyone know how to test this valve, or if it can be cleaned to return it to operable condition? Does the evap. system use engine manifold vacuum to scavenge fuel vapor from the fuel tank, and if so would it be a good idea to inspect the vacuum hoses in the system? Any help would be appreciated. It seems like every time I take my car to be serviced that the shops manage to damage an unrelated part of the car. I would really like to be able to perform most services myself as I know I take greater care than most techinicians who have worked on it in the past.

You need to find a good shop with Porsche experience and a PST tester.
If you aren't happy with your current shop, find another one and do some research with other Boxster owners.