As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IAH TO CMH - AFTER TAKEOFF, THE AIRCRAFT WOULD NOT PRESSURIZE AND AN OVERBOARD COOL MESSAGE WAS DISPLAYED ON EICAS. THE CREW RETURNED TO FIELD AND LANDED WITHOUT INCIDENT. MAINTENANCE TROUBLESHOT THE FAULT TO THE OVERBOARD EXHAUST SHUTOFF VALVE. THE VALVE WAS PLACED ON MEL AND THE PRESSURIZATION SYSTEM, OPS CHECKED GOOD.