As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR TURNBACK DUE TO UNABLE TO CONTROL PRESSURIZATION OF CABIN. CABIN ALT CLIMBING WITH OUTFLOW VLV FULL CLOSED AND BOTH PACKS IN AUTO. PASSING FL 240 GOT FLOW LIGHT. RETURN TO DEN. NO OVERHEAT INDICATION IN COCKPIT. FOUND DUCT CLAMP AND BOOT OFF OF LEFT PACK OUTLET TO WATER SEPARATOR OFF. REINSTALLED CLAMP WITH NEW LOCK NUT AND PRESS CABIN TO 4.5 PSID.