As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
RETURNED TO DEN DUE TO UNABLE TO PRESSURIZE CABIN. LANDED OVERWEIGHT. THE AUTO FLOW CONTROL VALVE (FOUR-INCH) FAILED TO CLOSE. REPLACED THE AUTO FLOW CONTROL VALVE, THEN PRESSURIZED AIRCRAFT USING BOTH APU AND ENGINE BLEED PRESSURE TO ENSURE CORRECT OPERATION.