As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CABIN ALT DISPLAY INDICATES ERRATIC AT FL 40 ALT. VARIED FROM 7.000 TO 10.000 WITH RATE FULLY PEGGED IN BOTH A CLIMB AND DESCENT. DIFF PRESSURE WAS CONSTANT. FOUND CABIN PRESSURE GAUGE FAULTY, REMOVED AND REPLACED PRESSURE GAUGE IAW REF AMM 21-33-01, OPS CHECKS OF SYS PER AMM 21-33-01-401 NO FAULTS NOTED.