As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING DESCENT, IN STBY MODE AND CABIN ALT SET TO 5230, CAB PRESSURE WENT AS LOW AS 3000 FT BEFORE NEEDING TO GO TO MANUAL. FAULTY OUTFLOW VALVE. FOUND DC SWITCHES OUT OF LIMITS. REPLACED VALVE ON A/C. REPAIRED VALVE IN SHOP WITH SERVICEABLE PARTS.