As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: CONTROL CABIN TEMPERATURE WAS UNCONTROLLABLE COLD, RAN QUICK REFERENCE HANDBOOK. AIR INTERRUPTED BACK TO ORD DUE TO THIS PRESSURIZATION ISSUE. EMERGENCY WAS NOT DECLARED AND NOT OVERWEIGHT FOR LANDING. REMOVED AND REPLACED TRIM AIR PRESSURE REGULATING VALVE PER AMM 21-62-01.