Know how you feel, is it a 250 petrol by any chance? Took 4 dealer visits to sort mine, updated engine software first time(car 4 weeks old), fix lasted 3 weeks. Diagnosis next time said thermostat faulty so that was changed, this fix lasted 3 days. Another visit diagnosed previous fault so case raised with Mercedes when I mentioned rejecting car due to fault. Last visit was for new software from Mercedes, this took 2 hrs to instal, so far so good (4 weeks ago). The dealers at the last visit did say they had 2 other cases of the same problem on the same engine.
Hi guys me too 4 days old 250 Petrol engine light came up. It really worried me as first ever brand new car. Took it to Mercedes dealer they did a software update which took about an hour and I off I went has been fine for around 10 days now but what a silly system!