As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: AUTO PRESSURE CONTROL SYSTEM #1 WAS UNSTABLE. THE OUTFLOW VALE WAS CYCLING CAUSING CABIN V/S, AND CABIN ALT, AND PRESSURE PSI CYCLING . V/S WAS CLIMBING UP AND DOWN 700 TO 350 FT/MIN. REMOVED AND REPLACED CABIN PRESSURE CONTROLLER IRW AMM 21-31-34-000-001-A AND 21-34-400-001-A.