What was found
ZONE TEMP IN CONTROL CABIN LIGHT ILLUMINATED. FOLLOWED QRH. LIGHT ILLUMINATED INTERMITTENTLY AND THEN REMAINED ON. SUPPLY DUCT TEMP WAS CONTROLLABLE; WE OPERATED IN THE OFF POSITION. GROUNDED - YES. PERFORMED PACK TEMP CONTROLLER BITE ON BOTH CONTROLLERS IAW B737-800 FIM 21-62 TASK 801 FOUND #1 CONTROLLER ZONE TRIM VALVE FAULT. REMOVED AND REPLACED VALVE 145 AFT CABIN TRIM AIR VALVE IAW B737-800 AMM 21-61-21. PERFORMED PACK TEMP CONTROLLER BITE ON BOTH CONTROLLERS IAW FIM 21-62 TASK 801. NO FAULTS. THIS CLEARS MEL 21-21G & MEL 21-18C.