A B737 FLC RETURNS LANDS WHEN UNABLE TO PRESSURIZE THEIR ACFT AT SLC; UT.

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ACFT ARRIVED LATE FROM PREVIOUS FLT DUE TO WX. MAINT LOG ITEM HAD BEEN ENTERED BY PREVIOUS CREW CONCERNING FLAP/GEAR WARNING SYS. DEPARTED OUR FLT AFTER REPAIRS COMPLETED. AFTER TKOF; A LOUD NOISE DEVELOPED IN NOSE AREA. AUTO FAIL LIGHT CAME ON INDICATING THAT WE WERE NOT PRESSURIZING PROPERLY. THERE WERE NO DOOR OPEN LIGHTS OR OTHER CAUTION OR WARNING LIGHTS. ELECTED TO RETURN TO DEP STATION. AFTER RETURN TO GATE; MECH RPTED THAT FORWARD ELECTRONIC BAY DOOR; OPENED FOR PRIOR MAINT; HAD NOT PROPERLY SEALED. DOOR WAS INSPECTED; ADJUSTED; AND RESECURED. DOOR WARNING PROX SWITCH WAS ADJUSTED AND TESTED. WE THEN AGAIN DEPARTED. IT WAS SUBSEQUENTLY DETERMINED THAT WE MAY HAVE LANDED APPROX 1000 LBS OVER MAX LNDG WT. THIS MAY HAVE BEEN OVERLOOKED WHILE ALL ACTIVITIES CTRED ON MAINT; NOISE; AND REDISPATCH PROCS. THIS SIT HAD CONTRIBUTING FACTORS OF FATIGUE AND FOCUS ON ALLEVIATING FURTHER DELAYS BY ALL THOSE INVOLVED. FURTHER REVIEW OF ALL FACTORS MAY HAVE UNCOVERED POSSIBLE OVERSIGHT/MISCALCULATION. FURTHER OCCURRENCES MIGHT BE AVOIDED WITH CAUTION/WARNING NOTE ADDED TO APPROPRIATE SECTION OF OPERATING MANUALS.