What was found
WHILE ON A CAT III APPROACH AT APPROXMATELY 3000 FT AGL, FAULT LIGHT ON THE HCP ILLUMINATED ALONG WITH A "NO A III" ON THE COMBINER AND HGS ANNUNCIATOR PANEL. RETURNED TO FIELD AND ACCOMPLISHED UNEVENTFUL LANDING. INTERROGATED HGS, FOUND FUALT CODE " FC-06 ", CLEANED HGS COMPUTOR TRAY FAN FILTER, IAW SMM ROCKWELL COLLINS PAGE 202 D, PERFORMED HGS RTS, IAW T/C 6343510. FAULT CODE EXTINGUISHED AND DID NOT RETURN. ACFT REMAINS CAT III STATUS, PLACARD IN FLIGHTDECK READS CAT III STATUS. RETURNED TO SERVICE.