The expansion tank contains a coolant level sensor. This device does not relieve you of the need for regular level control, but it is very helpful in case of a sudden drop, for example during a trip.
When the ignition is on and the engine is not running, the control lamp in the instrument cluster lights up halfway. At a normal coolant level, when the engine is started, the lamp goes out; if there is not enough coolant, on the contrary, it lights up brighter.
The control lamp is connected to a sensor in the expansion tank, the contact of which is closed by a float. If there is no liquid, the contact opens.
If the control lamp does not light up at all when the ignition is on, it may have burned out.
If the lamp continues to burn when the engine is running and the coolant level is normal, the sensor is faulty. In this case, when the plug is disconnected from the sensor, the lamp goes out. It is necessary to remove the sensor, check the float stroke and the contact of the connector. Replace sensor if necessary.