the check engin light came on after fueling???
Either you didn't tighten the cap properly (code/light will go away in a day or 2-3), or you got a bad tank of fuel, or it is not related to fueling-up (coincidence). I'm not familiar with Mercedes gas caps, but most model cars in that era or newer, you have to tighten a cap until you feel/hear 2-3 clicks. I might be wrong on the Mercedes product? just trying to be helpful.
Mr Goodwrench707 sounds spot on
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