BMW Repair: Check engine soon light came on., obdii scan tool, check engine soon light

Question
My wife's car is a 2001 BMW 530i. The other day she filled it with gas and drove home and forgot to screw on the gas cap and close the door. I screwed the cap on. But now 36 hours later, the "check engine soon" light came on. A friend tells me that's because we forgot to screw the gas cap on right away. True? How do I fix this situation so the light goes away.  

Answer
Bobby, a loose or missing gas cap will cause a check engine light. Usually happens a lot sooner than 36 hours. It could be another issue all together. You need to have the fault read out by a shop with a compatible OBDII scan tool. Should have an evap. system fault for major leak detected if the gas cap is the cause. Should cost an hours labor.
-Evan