My yellow check engine light came on today and mbrace/dealer seems to think its the gas cap...they told me to fill up and twist cap tightly. Tried and the light is still on

 

After you twist the cap tight, you still need to get someone with the fancy code tool to reset the check engine light switch.

I cannot swear that this is true for the W211, but it is for most.

 

I have had this happen on several different cars/ trucks. It will reset after a number of key on cycles. On my old Town and Country, it is over 20. I don't have any idea why it takes so long. Maybe it is to punish you for being careless. I read somewhere that over 90% of the check engine light problems are loose gas caps. Auto Zone will lend you a scan tool free of charge and you can reset it on the premisis. If it is another problem, the scanner will give you a place to start. The scanners are pretty reasonable and the codes are available on line. I have several cars with 125,000 plus miles so I bought one for $40 from Costco. So far three loose gas caps (friends and neighbors) and a defective speed control module(mine) that costs $300 to replace on my Maxima. Since speed control is pretty useless on highways here, I think I will pass.