What was found
DUAL FMC FAILURE. NORMAL OPERATION RESTORED AFTER APPROXIMATELY FIVE MINUTES. GROUNDED - YES. INITIAL EVALUATION, PERFORMED FMCS OPS TEST IAW B737-800 AMM 34-61-00-710-801. FMC-L/R SHOWS LH MMR "FAIL", ALL LEGS SHOW MMR "L". REMOVED AND REPLACED #1 MMR IAW B737-800 AMM 34-31-42-000/400-801. PASSES INSTALL TESTS. PERFORMED GPS OPS TEST IAW AMM 34-58-00-710-802 AND SYSTEM TEST AMM 34-58-00-730-802, BOTH TESTS PASS. PERFORMED MMR/ILS LRU REPLACEMENT TEST IN DFCS IAW AMM 22-11-00-740-815, NO FAULTS. RE-DID FMCS OPS TEST, NO FAULTS NOTED. A/C OK FOR CAT III APPROACH.