As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE FLYING CAT III APPROACH ILS INTO BOTH AIRPORTS NO AIII CAME UP ON HUD AND HAP AFTER INITIALLY SHOWING AIII. PASSING FINAL APPROACH FIX, AIII FLICKERED THEN NO AIII. TRIED RESETTING NO HELP. WEATHER BELOW MINIMUMS. RETURNED TO FIELD. LANDING UNEVENTFUL. INTERROGATED HGS, NO PRESENT FAULTS LEGS 1 AND 5 HAVE LAT GUIDE COMP AND MAG HDG COMP. REMOVED REPLACED HGS COMPUTER IAW MAINTENANCE 6343501, OPS CK GOOD ACFT REMAINS CAT III PLACARDED, CAT III. RETURNED TO SERVICE.