As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CABIN NOT CONTROLLABLE IN AUTO, STANDBY OR MANUAL MODE. RETURNED TO ORD. *S/D* NO OBVIOUS CAUSE OF PROBLEMS WERE DETECTED DURING CHECK OF PRESSURIZATION SYSTEM DURING MAINTENANCE. REPLACED CABIN PRESSURE SELECTOR PANEL AS PRECAUTIONARY.