What was found
DURING CRUISE, FMC P/RST ILLUMINATED, CA ND SHOWED VTK, MAP BLANKED OUT. QRH RUN FOR L FMC FAIL. TRANSFER SWITCH FOR FMC MOVED TO BOTH ON RIGHT. QRH COMPLETED, FLIGHT PROCEEDED WITH SWITCH IN BOTH ON RIGHT. GROUNDED - YES. INSPECTED AND FOUND LEFT FMC INOP. TRIED CYCLING CB - NO HELP. FMC L WAS MISSING AT MCDU. RMVD AND INSTALLED NEW LEFT FMC COMPUTER. CROSSLOADED SW FROM RT FMC COMP. ADJUSTED PERFORMANCE FACTOR REF B737-800 AMM 34-61-02/401. SYS OPS NML.