What was found
ON RECALL AFT CABIN ZONE TEMP LIGHT ILLUMINATED. WENT OUT ON MASTER CAUTION RESET. AFT CABIN TEMP REPORTED AS VERY HIGH. NOT CONTROLLABLE WITH TRIM AIR SWITCH ON. BECAME CONTROLLABLE WITH TRIM AIR SWITCH IN OFF POSITION. ACFT GROUNDED. PERFORMED PACK/ZONE TEMP CONTROLLER BITE TEST. FLAGGED ZONE TRIM VALVE ON NR 1 CONTROLLER. REMOVED AND REPLACED AFT CABIN TRIM AIR VALVE IAW AMM 21-61-21. OPS CHECKED GOOD. PASSED PACK/ZONE TEMP CONTROLLER BITE TEST AND LEAK CHECK GOOD.