What was found
AMBER DUCT OVHT ANNUNCIATOR ILLUMINATED ABOVE FL300 APPROXIMATELY 20 MINUTES INTO FLIGHT. CREW NOTICED THAT THE CREW TEMP INDICATOR READ 4 ON THE SCALE WITH SYSTEM IN MANUAL AND TEMP CONTROL SELECTED TO FULL COLD. CREW INITIATED THE APPROPRIATE ABNORMAL CHECKLIST PROCEDURES WITH CHANGE IN INDICATIONS. THE CREW DECIDED TO RETURN TO THE DEPARTURE AIRPORT AND CHANGE TO LOWER FL. AT FL260, THE CREW TEMP INDICATOR SCALE READING REDUCED AND THE DUCT OVHT ANNUNCIATOR EXTINGUISHED.