"One of many possibilities that doesn't throw a code is a big boost leak. If a hose pops off most of the boost and air leaks out instead of going into the engine.":thumbsup I don't think that's a valve, it's just a hose that wasn't installed tightly securely at the factory.
The scuff guard is generally called a splash shield. It's sometimes mistakenly called a skid plate but it's only a skid plate if you put in a metal replacement so that it provides some hard protection. As is, it provides water/sand/dust protection for the engine and belts, quiets it down, and improves aero. A metal replacement provides protection in case you hit something like road kill, ice chunks, or road debris. There's a FAQ article in the mk4 section which has more details.