Well, you might be lucky and it just be the gas cap, but there are many other things that cause the same symptoms. The fuel tank is sealed by the cap, but also by other components that can leak just like the gas cap can. There is an air pump that slightly pressurizes the fuel tank, a sensor that reads that pressure, and so on. They all cause the same symptoms when they fail. Reading the codes is the only way to determine what it actually is, other than randomly replacing components and hoping you hit the problem one.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue