When the pilot arms the system (prior to capture of either the localizer or glideslope) the flight control computers perform an extensive series of built-in tests. For a CAT III landing, all the sensors and all the flight computers must be in good health before the pilot receives an "AUTOLAND ARM" indication (generic indications that will vary depending on equipment supplier and aircraft manufacturer). If part of the system is in error, then an indication such as "APPROACH ONLY" would be presented to inform the flight crew that a CAT III landing is not possible.