A B737 FO RPTS OF AN FMC MALFUNCTION THAT LED TO A 30 NM COURSE DEV BEFORE IT WAS DISCOVERED. THE FMC FAILED TO ACCEPT ANY INPUT TO REPAIR THE SIT FROM THE FLC. THE FLC HAD BEEN DEVIATING FOR WX AND THOUGHT THAT THEY HAD REJOINED THE FLT PLAN RTE WHEN ATC QUERIED THEIR COURSE.

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DEPARTED MCO FOR LGA ON NORMAL PREFERRED FLT PLAN. EXTENSIVE DEVIATING AFTER TKOF DUE TO NUMEROUS TSTMS IN CENTRAL FLORIDA REGION. AFTER 20 PLUS MINS OF FLT I TURNED AND INTERCEPTED OUR FLT PLAN RTE CLOSE TO JAX (CRG). BOTH FLT MGMNT COMPUTERS SHOWED US ON COURSE; HOWEVER; ZJX INQUIRED ABOUT OUR 'PRESENT COURSE' BEING FLOWN. ON DOUBLE CHKING WITH RAW DATA NAV WE WERE ACTUALLY 30 MI S OF OUR FMC INDICATED POS. THE CAPT AND I ATTEMPTED TO UPDATE FMC POS MANUALLY BUT COMPUTER WOULDN'T ACCEPT INPUTS. FMCS ALSO WOULD NOT 'AUTO UPDATE POS' FROM THE IRS'S. THE INITIAL LATITUDE AND LONGITUDE POS INPUT WAS CHKED PRIOR TO THE FLT AND WAS CORRECT; NO DOUBT ABOUT THAT FACT BY EITHER PLT. ATC POINTED US DIRECT OUR NEXT RTE FIX AND THE FLT CONTINUED UNEVENTFULLY USING 'LOW TECHNOLOGY;' ACCURATE INPUT. THERE WAS NO CONFLICT OR PROB STATED BY ATC. FMCS WAS LOGGED INOP FOR MAINT AT LGA. FLC CHANGED ACFT. SINCE THE FMCS WOULDN'T UPDATE MANUALLY; AUTOMATICALLY; OR ALLOW FOR POS SHIFT I FEEL CERTAIN THAT PART OF THE SYS HAD FAILED.