As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE AIRCRAFT WAS DIVERTED TO MEM DUE TO A "CABIN ALTITUDE" WARNING LIGHT AT FL400, WITH ONLY A LT PACK EICAS MESSAGE. THE CREW DID ACCOMPLISH AN EMERGENCY DESCENT. CORRECTIVE ACTION: MAINTENANCE REPLACED THE LT PRSOV CONTROLLER & PRSOV AND A BROKEN LEADING EDGE DUCT COUPLING. PERFORMED ENGINE BLEED, SINGLE PACK CONFIDENCE & EXCESSIVE LEAK DOWN CHECKS. OK FOR SERVICE.