As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED. ORD-DTW FLIGHT RETURNED TO ORD ACCOUNT UNABLE TO PRESSURIZE. NO EMERGENCY DECLARED. TURNED ON LEFT PACK AND RECEIVED PRESSURIZATION FLOW LIGHT. UNEVENTFUL LANDING. LEFT PACK TURBINE DISCHARGE DUCT BOOT DISCONNECTED FROM TURBINE DISCHARGE PORT. RECONNECTED DUCT IAW AMM 21-51-01. OPERATIONAL CHECK GOOD.