As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PERFORMED AN AIR TURN BACK DUE TO ACFT CABIN WOULD NOT PRESSURIZE DURING CLIMB OUT. FLOW LIGHT ILLUMINATED. NO FAULTS ON EITHER CABIN PRESSURE CONTROLLERS. OUTFLOW VALVE OPERATION CHECKED GOOD. PRESSURIZED ACFT ON EACH PACK WITH NO EXTERNAL LEAKS FOUND. NO DUCT LEAKS TAIL COMPARTMENT. CABIN PRESSURE LEAK DOWN GOOD. CABIN SAFETY RELIEF VALVE CHECK GOOD. SINGLE ENGINE SECOND SEGMENT CLIMB SWITCH AMM 21-31 CHECKED GOOD.