On mine (190D) I had to replace the thermal sensor for the fan.
If your a Do it Yourselfer, try this
Start the engine, open the hood, and look for an electrical connector on your high pressure coolant intake.
Look for your temperature sensor, which is a thermistor.
Once you find it, check the connection by shorting it to ground. If the fan comes on, change the sensor.
If it doesn't, check the wiring in between the fan assembly, and the sensor.