When we receive a call for this issue, the most common cause is some sort of either internal or external leak. If you see any sort of coolant puddles forming near your unit, call a professional immediately. Also, look at your oil to see if it has changed texture or color. Your alarm should sound if the generator is low on coolant, but be observant just to be on the safe side.
A leak can also be due to a clogged radiator core. When a generator is working its thermostats are completely open. When a radiator is clogged it cannot allow the correct amount of coolant to flow through. Many times, that coolant will go through the overflow line as a result. Any time you suspect a coolant leak, call Reliable Power Systems immediately.