As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR TURNBACK, CABIN PRESS HORN WENT OFF AT 12,500 AND CABIN WOULD NOT HOLD PRESS. WOULD NOT RESTORE WITH STANDBY OR MAN. REMOVED AND REPLACED AIR CYCLE MACHINE LT PACK AND AIR CONDITION RELAY CURRENT HISTORY: ATA 21 SEVERAL WRITE-UPS IN LAST THREE DAYS, MORE HISTORY ON LT PACK.