What was found
AIRCRAFT DEPARTED KELP HEADING TO KAUS. ABOUT 10 MINUTES AFTER TAKEOFF, THE CAPTAIN NOTICED THE FMS SCREEN HAD GONE BLANK. ATTEMPT TOP RE POWER WAS MADE WITH NO EFFECT. THE CAPTAIN DECIDED TO RETURN TO ELP. ATI MAINTENANCE AND AVIONICS MET THE AIRCRAFT AND VERIFIED THE FMS UNIT WAS NOT POWERING BACK ON. THE UNIT WAS PULLED AND FOUND TO BE EXTREMELY HOT. AFTER COOLING THE UNIT WAS RE INSTALLED AND IT POWERED UP - COLLING FAN NEVER TURNED ON AND THE UNIT GOT HOT AGAIN. A REPLACEMENT FMS WAS INSTALLED AND WHEN POWERED UP, THE COOLING FAN IMMEDIATELY TURNED ON. IT WAS DETERMINED THAT THE TEMP SENSOR OR THE FAN WAS INOP IN THE UNIT REMOVED. THE REPLACEMENT FMS WAS LEFT POWERED UP FOR TWO HOURS WITH NO OPERATIONAL ISSUES NOTED. THE AIRCRAFT WAS THEN RETURNED TO SERVICE.