Mercedes-Benz W202 coolant warning light on although there is coolant
Why is the Mercedes-Benz W202 coolant light on although there is enough coolant?
Most often the fault is in a sensor and not in the coolant level. The cause may be the expansion tank level sensor, the windscreen washer fluid sensor or the electric fan control box. The washer fluid sensor has been suspected of being on the same circuit, so check both sensors and their wires and connectors.
How does the coolant level sensor work and why does it stick?
The sensor's glass tube holds two metal contacts that close when the expansion tank float with its magnet is near. The float or the sensor can stick because of limescale or dirt. First try tapping the tank near the sensor, then clean the tank or replace the sensor. The sensor is in the bottom of the tank, so some of the coolant has to be drained when replacing it.
Can the fan control box turn on the coolant light?
Yes. The control box is inside the inner wing, and moisture can oxidise its connectors or housing. The fault shows up as the light and, for example, the fan not starting as the temperature rises. On a 220 CDI, fault code P1636 indicated an interruption in the fan control. Measure the sensors first, then check the box's connection and condition.