As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SEA - NO CAT III INTO SEA, WOULD ARM THEN GO AWAY ON APPROACHES. DIVERTED TO PDX. ALSO TRIED CAT III IN PDX AND GOT THE SAME RESULTS. FOUND NUMEROUS FMS2-2 FAULTS IN HGS FAULTS, REMOVED AND INSTALLED NEW NR 2 FMS. PERFORMED OPERATIONAL TEST OF NR 2 FMS IAW Q400 TC 6346101, ALL CHECKS GOOD. AIRCRAFT RETURNED TO SERVICE.