When the Check Engine light is on, the cars computer will record & store a Fault Code.
This Code can be retrieved with a Scantool.
You will need to find someone ( M-B dealer if necessary ) to do this.
They should then be able to tell you what the problem/Cure is.
And before anybody tells this new member to build a " Code Retrieval Tool ", I don't get the impression it is within his/her capabilities !