What was found
ATTEMPTED MLP IN NR 1 FMC APPROXIMATELY 1 HR INTO FLT, RECEIVED A ROUTE MOD UNABLE MESSAGE IN SCRATCHPAD. NR 1 FMC SCREEN WENT BLANK AND NR 1 MAP DISPLAYED VTK W/NO NAVIGATION DATA. NR 2 FMC MAP SUBSEQUENTLY FAILED BEFORE WE COULD RUN QRH. NR 2 FMC RETURNED FOLLOWED BY NR 1. HOWEVER RECEIVED SAME RT MOD UNABLE MESSAGE WHETHER WE TRIED MLP IN NR 1 OR NR 2 FMC. BOTH FMCS SUBSEQUENTLY FAILED AGAIN BUT QUICKLY RETURNED NR 2 FIRST THEN NR 1. FINALLY ABLE TO INPUT MLP. ACFT GROUNDED. LT FMC LOGGED PGM FAULT & LOC DEV SSM FAULT. PERFORMED FIM 34-61, TASK 807 & 823, ALL TESTS PASSED AND FMC LT BITE CHECK GOOD, REF FIM 34-61, TASK 801. RT FMC LOGGED PRESSURE POSITION LAT, RATE AND LOCATION DEV SSM FAULT. PERFORMED FIM 34-61, TASK 824 & 823. ALL TESTS PASSED, RT FMC BITE CHECK PASSED, REF FIM 34-61, TASK 801.