While manufacturers use different types of coolant-level sensors, the simplest is a float that activates a switch. If the coolant drops below the designated level, the switch opens, sending a signal to the indicator light. Manufacturers install these switches either in the coolant recovery tank or in the radiator. Regardless of the type of sensor, it should cause the light to illuminate if the level drops.
Sometimes, the indicator sensor malfunctions. If a visual inspection of the coolant level reveals it is within the proper limits when the light is on, the problem is usually with the sensor. Floats operate under hot, wet conditions, and they sometimes become stuck. A dirty float or one with disconnected electrical connections cannot work correctly but can be rectified by cleaning or reconnecting the wire. However, sometimes it is necessary to replace the unit.
Replacing the switch is within the repair capacities of many vehicle owners. It requires draining the coolant from the vehicle. Coolant is toxic to pets and children and requires disposal in a safe manner. It is sometimes necessary to remove other parts to access the sensor. When the sensor is located in the coolant recovery tank, it is often necessary to replace the entire tank. After completing the process, it is essential to refill the system with a 50/50 mix of antifreeze and water.