What was found
THERE IS AN ISSUE WITH THE CABIN TEMP CONTROL SYS. OVERHEAD TEMP INDICATOR AND CONTROL PANEL IS NOT AGREEING WITH THE REPORTED TEMPS, MAKING THE CABIN TEMP- ESPECIALLY AFT VERY DIFFICULT TO PROPERLY CONTROL. AFT CABIN IS INDICATING 22C AFT SUPPLY IS INDICATING 12C THIS IS HIGHLY UNUSUAL, ESPECIALLY AFTER BEING STABILIZED IN CRUISE FOR OVER 2 HRS. ACFT GROUNDED. PERFORMED BITE CHECK. LT PASS. RIGHT SHOWS MESSAGE TEMP CONTROL VALVE 21-51266 AND 21-51267. PERFORMED FIM TASK 21-62, TASK 831. CHANGED RT TEMP CONTROL VALVE AMM 21-51-10. REPEATED BITE. PASS, LEAK AND OPS CHECK GOOD, REF B737-900, IPC 21-51-10-04, ITEM 90.