As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLT 711 FOR DEN TO TOL. DURING CLIMB OPERATION FROM DEN THE AIRCRAFT WAS UNABLE TO MAINTAIN CABIN ALTITUDE BELOW 10000 FEET WITH ONE PACK OPERATION. AIRCRAFT RETURNED TO DEN WHERE IT LANDED WITHOUT INCIDENT. POST FLIGHT INSPECTION REVEALED LEFT ACM INOP. REMOVED AND REPLACED LEFT ACM AND LEFT PACK SHUTOFF VALVE, OPS CHECK GOOD. NO OTHER DEFECTS NOTED. AIRCRAFT WAS RETURNED TO SERVICE.