Low water sensors are more commonly known, or referred to, as coolant level sensors. The purpose of these sensors is to alert the driver of low coolant level in the engine cooling system. When coolant level in the engine is low, the cooling system cannot adequately remove excess heat from the engine. This causes the engine to overheat which will lead to eventual engine failure. The most common results of engine overheating include warped or cracked cylinder heads, and blown head gaskets.
The typical coolant level sensor has two probes that, when inserted into engine coolant, pass electricity between them. This creates a complete electrical circuit. When coolant level falls below the probes, the circuit is broken and the low coolant level warning lamp is illuminated. This alerts the driver to have the cooling system checked.
There are basically two symptoms of a failed coolant level sensor.
Low Coolant Warning Light always on
Since the Low Coolant Warning Light is illuminated when the lamp circuit is electrically open, the sensor could go open internally regardless of actual coolant level. This would cause the coolant light to always be on. The necessary repair here would be to replace the sensor.
Low Coolant Warning Light does not illuminate on low level
If the coolant level sensor is electrically shorted internally, the coolant level circuit will never go open when coolant level falls too low. This prevents the Coolant Level Warning Light from illuminating and alerting the driver to a potentially hazardous situation. If the cooling system isn’t checked on a regular basis, engine damage may occur.
Have a trained technician like those at YourMechanic diagnose the condition. If the sensor needs replacement, they can do that. If another issue is the cause, they can fix that, too.