As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMB, CREW WAS UNABLE TO PRESSSURIZED AIRCRAFT. MAINTENANCE FOUND AFT OUTFLOW VALVE IN OPENED POSITION. EMERGENCY DECLARED. LANDING UNEVENTFUL. AIRCRAFT GROUNDED. ACCOMPLISHED FIM TASK 21-31-00-810-806. REMOVED AND REPLACED LEFT ELECTRONIC CONTROL UNIT OF AFT OUTFLOW VALVE IAW AMM 21-31-03. OPS CHECKED NORMAL.