A B737-700 FMC VNAV MALFUNCTION INDICATING EXTENDED FLAPS AND INSUFFICIENT DESTINATION FUEL FORCED THE FLT TO RETURN TO DEP ARPT.

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AFTER WE RETRACTED THE FLAPS; COMPLETED THE AFTER TKOF CHKLIST; CLBED UP THROUGH 3000 FT. WE TRIED TO ENGAGE VNAV -- IT DID NOT ENGAGE. I IMMEDIATELY NOTICED THAT THE AIRSPD BUG DID NOT AUTOMATICALLY ADJUST FROM FLAPS UP SPD TO 250 KIAS. I TOOK A LOOK AT THE CAPT'S CDU DISPLAY SCREEN WHICH WAS DISPLAYING ECONOMY CLB PAGE AS IT IS SUPPOSED TO BE. I THEN CHANGED MY FMC TO DISPLAY THE ECONOMY CLB PAGE. IN THE LINE SPACE THAT USUALLY DISPLAYS 'SPD RESTR' '250/10000;' IT READ 'SPD RESTR' '212/FLAPS.' WE CONTINUED TO CLB THROUGH 10000 FT. AGAIN; THE 'SPD RESTR' WINDOW WHICH WAS SUPPOSED TO READ '--/---' AND THE SPD CHANGES TO TARGET SPD AUTOMATICALLY FOR THE CLB. THERE WAS NO CHANGE AND BOTH CDU'S STILL READ '212/FLAPS' UNDER 'SPD RESTR.' WE WERE CLRED DIRECT TO ABC AND CONTINUED TO CLB TO FL380. LNAV WORKED AS IT SHOULD. THE CAPT ENGAGED THE AUTOPLT; ASKED ME TO FLY WHILE SHE MANAGED THE PROB WITH MAINT CTL. THE CAPT BEGAN TO LOOK THROUGH THE PAGES IN THE FMC FOR INCORRECT ENTRIES OR UNEXECUTED ENTRIES. AFTER I GOT MAINT CTL ON 129.9; MAINT HAD A DIFFICULT TIME UNDERSTANDING THE ISSUE AND SUGGESTED SWITCHING THE FMC TO BOTH ON 1 AND BOTH ON 2. WE ACCOMPLISHED THIS. THERE WERE NO CHANGES TO EITHER FMC'S. BOTH FMC'S CONTINUED WITH THE SAME MIS-READINGS. THE FMC LIGHT ILLUMINATED ON THE L FORWARD PANEL. HOWEVER; NO MESSAGE APPEARED IN THE SCRATCHPAD ON THE FMC. WHILE LOOKING FOR DISCREPANCIES AND COMPARING THE 2 CDU'S; THE FUEL FOR ARR IN ZZZ2 DISPLAYED 4.9 ON BOTH CDU'S. THE TARGET SPDS FOR ALL THE LEGS WERE COMPLETELY WRONG. INFO THAT IS NORMALLY AVAILABLE WAS MISSING ON SEVERAL PAGES. WHILE TALKING WITH MAINT; WE WERE ASKED TO STAND BY OR HOLD ON; WE THEN REALIZED THAT WE WERE OUT OF VHF RADIO RANGE; AND THEY WERE NO LONGER ON THE FREQ. WE CALLED COMMERCIAL RADIO AND ASKED FOR A PHONE PATCH WITH DISPATCH/MAINT. WHILE CHKING FOR POPPED CIRCUIT BREAKERS; CHKLISTS; AND MANUALS FOR INFO AND WAITING FOR MAINT TO RETURN TO THE FREQ. MAINT HAD NO SUGGESTIONS EXCEPT A COMPLETE 'REBOOT.' I ASKED IF THERE WAS ANYTHING WE COULD DO INFLT. WE DID NOT GET AN ANSWER TO THAT QUESTION. WE ASKED IF ANYONE IN FLT OPS HAD ANY SUGGESTIONS. NO SUGGESTIONS CAME BACK. OUR FIRST THOUGHT WAS MAYBE WE HAD OVERLOOKED SOMETHING IN OUR PROGRAMMING AND THAT THE MESSAGE '212/FLAPS' COULD BE SOMETHING SIMPLE THAT WAS LEFT OUT OR NEEDED ADDING. AS WE CROSSED ABC; THE FUEL FOR ARR IN ZZZ2 FELL TO 1.1 AND THE 2 CDU'S BEGAN TO DISPLAY DIFFERENT INFO. MAINT; AT THE SAME TIME; RECOMMENDED TURNING AROUND AND RETURNING TO ZZZ. WE THEN ADVISED ATC THAT WE NEEDED TO RETURN. WE TURNED AROUND WITH A CLRNC TO ZZZ. AFTER INPUTTING THE ARR AND APCH INTO THE FMC; THE DISCREPANCIES BTWN BOTH FMC'S WERE INCREASING. COMING ABEAM CDE; THE MAP DISPLAY SHOWED 2 PARALLEL RWYS FOR ZZZ; EVEN THOUGH WE PROGRAMMED THE ILS INTO THE FMC'S. WE ALSO ADVISED ATC THAT WE WERE UNABLE TO COMPLY WITH LAHSO DUE TO AN OVERWT LNDG. WE LANDED WITHOUT INCIDENT. THE MAINT COMPUTER TECHNICIAN WAS UNABLE TO FIND ANY FAULT/PROBS IN THE SYS. BOTH OF THE FMS COMPUTER SYS WERE FINALLY REBOOTED AND WE HAD NO FURTHER PROBS ON THE FLT.