What causes check engine light on 2002 BMW 325CI?

[MUSTANGGT351]

code p0455 evap leak. gas cap is ok and tight.

 

[thebax2006]

The gas cap can be faulty internally, you need to test it with a vacuum testing machine, (Miller pump)
You may also have a cracked vapor canister or a solenoid bad on the canister vent.
Bad hoses may also be the problem but not as common as a cracked canister, solenoid, or gas cap.
Time for the dealer!