I had a similar problem with an ML320 Benz. A sensor that is intermittently bad will throw the check engine light and then after you start the engine about three time the light will clear only to come on again the next time the sensor fails. Did they tell you what codes are being thrown? Or what sensor they changed? Some codes can relate to multiple problems/sensors which can make changing the right sensor more of a challenge. you might need to find a mechanic more experienced with these engines or take it to a Mercedes dealer or private mercedes mechanic. Probably not an option as you want this covered under warrenty?