As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
EICAS MESSAGE CABIN AUTO INOP. ABLE TO CONTROL CABIN ALTITUDE OF 10,000 FEET. DIVERTED ORD. *S/D* AIRCRAFT WAS UNABLE TO MAINTAIN PRESSURIZATION DUE A MECHANICAL FAILURE OF THE OVERBOARD EXHAUST VALVE. SHOP DISASSEMBLY AND INSPECTION OF THE OVERBOARD EXHAUST VALVE DETERMINED THAT THE CLUTCH ASSEMBLY WAS SLIPPING. THIS SLIPPAGE ALLOWED THE VALVE FLAPPER TO UNSEAT, WHICH RESULTED IN CABIN DEPRESSURIZATION.