As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
UNABLE TO CONTROL PRESSURIZATION WITH BOTH PACKS ON IN CLIMB. AT 21000 CABIN ALTITUDE WAS RISING 100 FPM WITH THE FORWARD AND AFT OUTFLOW VALVE FULLY CLOSED. RETURNED TO SEA. *S/D* RIGHT HAND HEAT EXCHANGER AND LEFT HAND PACK SWITCH DEFECTIVE. REPLACED UNITS. PRESSURIZED CABIN TO 35000 FLT AND OPERATION CHECK OK.