I have adapted this from my original tutorial for the BMW M30 engines. They both use Bosch systems and the valve is identical. What happens is that at startup the valve stays open to draw fumes caught in the charcoal canister. The fumes are there because they evaporate from the fuel tank. This is why your gas cap is usually checked at emissions inspections and why the manual says you may have a temporary check engine light when you fill up with the car on. The only place evaporating gas can go is the charcoal canister which is then pulled into the intake at startup.
This is pretty much always the P0455 code.
Read the rest of this fine tutorial w/ photos: How to test and repair your EVAP valve