As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW RECEIVED HIGH ALTITUDE EICAS MESSAGE. CREW DECLARED EMERGENCY AND RETURNED TO FIELD. MAINTENANCE CHECKED PRESENT LEG MESSAGES AND HAD A WARNING FOR "CABIN ALT HIGH" WITH NO LRU'S AT FAULT AND NO CORRELATED MX MESSAGES. MAINTENANCE REPOWERED THE ACFT AND PERFORMED A GROUND RUN TO PRESSURIZE THE CABIN; SYSTEM OPS CHECK NORMAL. MAINTENANCE WAS UNABLE TO DUPLICATE DISCREPANCY ON GROUND. THERE IS NO PRIOR HISTORY AND DISCREPANCY HAS NOT REPEATED THIS DATE 12-12-2008.