The most obvious cause for a low coolant level is either an external or internal leak. Close attention should be paid to notice any visible puddles of coolant during weekly inspections of the unit(s). The color of the coolant varies by manufacturers and may look like red-dyed diesel fuel. Oil should also be inspected for any signs of a change in color or a milky texture. Hoses should be inspected for "crusties" – the sign of coolant seeping and the additives drying up at the connection.
While many generators are equipped with this alarm, few generators have a dedicated alarm indicator for low coolant. Commonly, this alarm will be tied into a high coolant temp shutdown. If the generator is equipped with an "Approaching High Coolant Temp Alarm" or "High Coolant Temp Pre Alarm," you can determine which fault caused the shutdown. If the high temp shutdown is activated without a pre alarm, it is most likely responding to the coolant level.
Internally plugged radiator cores will also cause low coolant level shutdowns. When the generator is under load, the thermostats open completely and the radiator cannot allow the proper amount of flow through the core. The coolant has to go somewhere, and so purges through the overflow line. As the engine cools off and the thermostat closes, the level drops and activates the low coolant level shutdown. This also occurs when float switch type coolant level sensors are used and the lines are plumbed to the top and bottom radiator tanks. When the thermostats open, the path of least resistance is through the float switch lines and the flow causes the float to drop and shutdown the engine. The thermostat will not open enough to cause this during regular weekly running of the generator. The generator will have to be tested under load to cause the thermostats to open completely. A full load test with an external load bank is the only accurate way to check a cooling system.