What was found
PILOT SELECTED ENVIRONMENTAL SYSTEMS TO AUTO PRIOR TO FLIGHT AND RECEIVED A CAUTION LIGHT ON PANEL THAT THE SYSTEM WAS NOT FUNCTIONING. THE SYSTEM WAS CYCLED ON AND OFF THEN AN INCREASE IN POWER WAS APPLIED AND THE CAUTION LIGHT EXTINGUISHED INDICATING NORMAL. FURTHER FLIGHTS THAT DAY WERE UNEVENTFUL. MAINTENANCE TROUBLESHOOTING VIA THE MANUAL SWITCH FROM FULL COLD THEN FULL HOT RESULTED IN A STUCK IN HOT VALVE. THE TEMP VALVE WAS REPLACED AND SYSTEM FUNCTIONED FINE. AIRCRAFT TTSN 4650HRS,CYC 3286, THE ORIGINAL VALVE WAS REPLACED AT 3837 HRS 2662 CYC. THE ONE REPORTED FAILED AT 810 HRS AND 622 CYC.