As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMB OPERATION WITH THE RT PACK ON DMI. THE LT PACK WOULD NOT MAINTAIN CABIN ALTITUDE. ACFT LEVELED OF AT 18,000 FT THE CABIN CONTINUED TO CLIMB AT A RATE OF 500 FT/MIN WITH CARGO HEAT OUTFLOW VALVE AND OUT FLOW VALVE CLOSED. THE ACFT RETURNED TO DEPARTURE WHERE IT LANDED WITHOUT INCIDENT. POST FLIGHT INSP WAS PERFORMED. R & R RT PACK SHUT VALVE, R & R LT FLOW CONTROL VALVE. PERFORMED OPS CHECKS, NO OTHER DEFECTS NOTED. ACFT WAS RETURNED TO SERVICE.