What was found
(CAN) GROUND PERSONNEL OBSERVED SMOKE EMANATING FROM ACFT AND ADVISED TOWER. ARFF PERSONNEL RESPONDED AND DISCOVERED THAT AN ONBOARD GROUND HEATER, HAD OVERHEATED, BUT WAS NOT ON FIRE. A SUBSEQUENT INSPECTION REVEALED THAT WHEN THE HEATER STARTED SMOKING, IT WENT INTO A COOLANT OVER-TEMP MODE AND SHUTDOWN. THERE WAS NO DAMAGE TO THE ACFT. THE HEATER WAS DISCONNECTED AND THE ACFT RETURNED TO SERVICE.