As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: FLIGHT RETURNED TO ORD ON ACCOUNT OF PRESSURIZATION PROBLEM. AN EMERGENCY WAS DECLARED. MAINTENANCE ACCOMPLISHED FIM TASK 21-62-00-810-849, FOUND STANDBY TEMPERATURE CONTROL VALVE STAYS OPEN WHEN PACK IS RUNNING AND ALL SENSE LINES ARE DISCONNECTED. REMOVED AND REPLACED TEMP CONTROL VALVE IAW AMM 21-51-10. OPS CHECK GOOD NO LEAKS.