What was found
LOSS OF BOTH PACKS CAUSED RESULTANT PRESSURIZATION PROBLEM. ALTHOUGH RT PACK WAS DEFERRED IN STANDBY MODE PER MEL AT THE TIME OF THE EVENT, THERE IS NO RECORD OF ANY DEFECT OR MAINTENANCE ACTIVITY AGAINST THE LT PACK, PRIOR TO OR AFTER THE EVENT. RT PACK COMPRESSOR OUTLET TEMP SENSOR AND RT PACK RAM AIR INLET ACTUATOR WERE BOTH REPLACED. MAINTENANCE ALSO REPLACED LT BLEED ISOLATION VALVE DUE TO DAMAGED BUTTERFLY SEAL. HOWEVER, NONE OF THE AVAILABLE DATA EXPLAINS LOSS OF CABIN PRESSURIZATION.