What was found
DURING PRE FLIGHT AND TAXI OUT L FMC FAILED AND RETURNED. AFTER 6 HOURS IN FLIGHT L FMC FAILED AGAIN. GROUNDED - YES. REMOVED AND REPLACED LEFT HAND FMC IAW B737-900 AMM 34-61-02/401, GROUND FUNCTIONAL TEST FOR THIS ETOPS SIGNIFICANT SYSTEM IS GOOD IAW FMCS PERFORMANCE FACTORS - ADJUSTMENT - AMM TASK 34-61-00-800-801 AND FLIGHT MANAGEMENT COMPUTER SYSTEM - OPERATIONAL TEST IAW AMM 34-61-00-710-801. ALSO CROSS LOADED ALL REQUIRED SOFTWARE IAW AMM 34-61-00/201 - REFERENCING B737-900 IPC 34-61-02-02B, 34-61-02-11, AND 34-61-02-99.