The engine may develop a stalling and/or no start condition due to a failed crankshaft position sensor.Google+
Car Problem Reports
Mercedes-Benz SLK350 Engine Performance Problems Due to Crankshaft Position Sensor Failure
Mercedes-Benz SLK350 Problem
Visitor, 2005 Mercedes-Benz SLK350, 35,500 mi
Sometimes need to turn the key two or three times to start the car. The crank shaft sensor has been replaced and the problem still occurs. The car always starts by the third try. Mercedes dealer could not find any error codes and said until the car will not start at all they cannot fix the problem.