As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
RETURNED TO DEPARTURE, BOTH CABIN AUTO PRESSURE CONTROLLERS FAILED IN FLIGHT. AIRCRAFT WAS GROUNDED. AUTO1 CABIN ALT BITE TEST SHOWS TO REPLACE THE CABIN PRESSURE CONTROL UNIT IAW 21-30-00-815-802 FIM. REMOVED AND REPLACED THE NR 1 AND 2 PRESSURE CONTROLLERS IAW AMM 21-31-02, AND REMOVED AND REPLACED OUTFLOW VALVE ACTUATOR MOTOR IAW MM 21-31-05.