As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
EICAS "OUTFLOW VALVE L" AT 6000 FT ON TAKEOFF. PASSING THROUGH 11,500 FT "CABIN ALT" EICAS HORN AND LIGHTS. COULD NOT CONTROL PRESSURIZATION RETURNED TO DEPARTURE. REPLACED LT OUTFLOW VALVE ACTUATOR ASSY IAW MM. OPS TEST PASSED. MESSAGE CLEARED.