As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT RETURNED TO CVG BECAUSE THE AIRCRAFT FAILED TO PRESSURIZE. FLOW LIGHT AND NO MODE LIGHT ILLUMINATED WITH CR POWER SELECTED. OUTFLOW VALVE DROVE FULL CLOSED AND NO AIR FLOW FROM PACKS. REPLACED NUMBER 1 ENGINE AUTO PRESS DIFF SWITCH. CHECKS OK.