What was found
PDIS 791 21-30 4670 04SEP21 CPR OPEN U CABIN ALTITUDE WARNING OCCURRED AT CRUISE FL 390 WITH CABIN ALTITUDE CONTROL IN AUTO 2. BEGAN AN IMMEDIATE EMERGENCY DESCENT TO 16,000 WHILE RUNNING QRC AND QRH CHECKLISTS. CABIN ALTITUDE SEEMED TO COME DOWN SLOWLY WITH OUR DESCENT. AT 16,000 SWITCHED TO AUTO 1 WHICH SEEMED TO HELP INCREASE THE CABIN ALTITUDE DESCENT RATE. EVENTUALLY STABILIZED AT 6,000-7,000 CABIN ALTITUDE WHILE IN LEVEL FLIGHT AT 16,000. MEL 04SEP21 02:35 CPR U RELEASED FOR SERVICE PER MEL : 21-31-01-A CATEGORY : C EXPIRATION : 14SEP21 "MEL 21-31-01-A: _ RELEASED FOR SERVICE PER MEL : 21-31-01-A CATEGORY : C EXPIRATION : 14SEP21 C/7284928 UPDT 04SEP21 02:51 CPR U "MEL 21-31-01-A: NUMBER 2_ AUTO CABIN PRESSURE CONTROL SYSTEM - INOP. PART 04SEP21 02:54 MC1 U IDENTIFIED PARTS MPN: NULL UPDT 04SEP21 02:54 CPR U AMT PUT IN WORK IN ERROR. REMOVED FROM IN WORK. FACT 07SEP21 23:32 LFT REMOVED AND REPLACED AUTO PRESSURE CONTROLLER #2. IAW 757 M/M 21-31-02-4. OPS CHECK GOOD.