As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT FL310, THE CABIN PRESSURE BECAME UNSTABLE AND THEN COMPLETELY FAILED. CREW FOLLOWED COM PROCEDURES AND DESCENDED TO 8000 FT. MASKS DID DEPLOY IN THE CABIN. FLIGHT RETURNED TO MSP AND LANDED UNEVENTFULLY. MAINTENANCE FOUND THE OUTFLOW VALVE OUT OF RIG AND REPLACED THE DIFFERENTIAL PRESSURE SENSOR. OPERATIONAL CHECK GOOD. AIRCRAFT GOOD FOR SERVICE.