As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT FL380 CABIN ALT WARNING AND HORN WENT ON, CABIN ALT APPROX 12000 FT MODE SELECT AUTO 1 CREW ELECTED TO PERFORM AN AIR TURN BACK. AIRCRAFT LANDED WITHOUT FURTHER INCIDENT. AIRCRAFT GROUNDED BY MAINTENANCE . MAINTENANCE DEFERRED PRESSURIZATION SYSTEM CONTROLLER #1 IAW MEL 21-31-1A. AIRCRAFT RETURNED TO SERVICE.