As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ACFT RETURN TO DEPARTURE DUE TO CABIN ALT HIGH EICAS MESSAGE WHEN CLIMBING OUT OF 28,000 FT. CABIN ALTITUDE INDICATED 8000 FT AND OXYGEN MASKS DROPPED. CABIN ALTITUDE INDICATOR NEVER WENT ABOVE 8000 FT AND IMMEDIATELY STARTED DOWN DURING DESCENT. R & R ALTITUDE PRESSURE SWITCH IAW AMM 35-21-02. PERFORMED FUNCTIONAL CHECK OF ALTITUDE PRESSURE SWITCH IAW AMM 35-21-02 SYS. OPS CHECKED NORMAL.