As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT APPROXIMATELY 12K, THE CABIN PRESSURE CONTROL WENT FROM AUTO TO STANDBY. FIVE MINUTES LATER AND AT 23K, RATE OF CLIMB PRESSURE OF AIRCRAFT INCREASED TO APPROXIMATELY 1000 FEET BELOW SEA LEVEL. CABIN DIFFERENTIAL PRESSURE WAS IN YELLOW BAND BUT DID NOT EXCEED RED LINE. *S/D* MAIN OUTFLOW VALVE FAILED. REMOVED AND REPLACED MAIN OUTFLOW VALVE.