As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DEN - FLIGHT 4104 - THE FLIGHT RETURNED TO FIELD WHEN DURING THE CLIMB THRU FL230, THE CABIN ALTITUDE CAUTION LIGHT CHANGED TO A CABIN PRESSURE WARNING LIGHT. THE CREW PERFORMED AN EMERGENCY DESCENT. THE FLIGHT CREW NOTED NO CONTROL OF CABIN PRESSURE. MAINTENANCE INSPECTED THE AIRCRAFT AND REPLACED THE LEFT PACK FLOW CONTROL VALVE. OPERATIONAL CHECKS WERE GOOD AND THE AIRCRAFT WAS RETURNED TO SERVICE.