As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON FLIGHT 3960 DURING CLIMBOUT THE CREW REPORTED THE AIRCRAFT WOULD NOT PRESSURIZE. THE CREW TRIED AUTO AND MANUAL MODE. NO EICAS MESSAGES RELATED TO PRESSURIZATION. THE CREW ELECTED TO RETURN TO ORD. THE AIRCRAFT LANDED WITHOUT INCIDENT. REMOVED AND REPLACED THE ELECTRIC PNEUMATIC OUTFLOW VALVE IAW AMM 21-31-03. THE OPERATIONAL CHECK WAS GOOD. NO FURTHER DISCREPANCIES NOTED. THE AIRCRAFT WAS RETURNED TO SERVICE.