ON DEP FROM ONT FLT CREW OF B733 EXPERIENCE UNSAFE GEAR LIGHT AFTER GEAR RETRACTION. CONTINUED TO FLY SID DUE TO MOUNTAINOUS TERRAIN WHILE TROUBLESHOOTING THE PROB. DETERMINED TO BE INDICATING SYS.

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WHEN THE CAPT CALLED FOR LNDG GEAR UP AFTER ROTATION; I DID SO AND THE RETRACTION WAS NORMAL. HOWEVER; I NOTICED THAT THE RED L GEAR UNSAFE LIGHT REMAINED ON. WE RETRACTED FLAPS ON SCHEDULE AND CONTINUED ON THE SID. AFTER MOVING THE LNDG GEAR LEVER TO OFF; THE LIGHT REMAINED ON. THE CAPT CALLED FOR A GEAR CYCLE; WHICH RESULTED IN ALL OF THE RED UNSAFE LIGHTS GOING OUT; AND 3 GREEN LIGHTS ON EXTENSION. WHEN WE RETRACTED THE GEAR; HOWEVER; IT RESULTED IN THE L GEAR UNSAFE LIGHT REMAINING ILLUMINATED WITH ALL OTHER LIGHTS OUT. THE CREW FELT NO VIBRATION AND HEARD NO SOUNDS TO INDICATE THAT THE GEAR WAS NOT RETRACTED. BECAUSE OF THE TERRAIN IN ONT WE CONTINUED ON THE SID AND CLB; AND CONSULTED THE QRH (EMER CHKLIST). THERE WAS NO LNDG GEAR CHKLIST THAT ADDRESSED OUR PARTICULAR PROB; AS THEY ARE FOR PROBS WITH GEAR EXTENSION (NOT RETRACTION) OR PROBS WITH THE GEAR LEVER UPON RETRACTION (WHICH WAS NOT AN ISSUE IN OUR CASE). AFTER BOTH CREW MEMBERS CONSULTED THE QRH; WE DECIDED THAT IT WAS DEFINITELY SAFE TO CONTINUE (NO DIFFICULTIES WITH PRESSURIZATION; NO WARNING HORNS; ETC). THE CAPT CALLED DISPATCH WHO PUT US THROUGH TO MAINT AND THEY AGREED THAT IT WAS JUST AN INDICATION PROB. HOWEVER; BECAUSE THERE'S ONLY CONTRACT MAINT IN SMF; WE WERE DIVERTED TO OAK. THE FLT ATTENDANTS AND PAX WERE INFORMED. WE CHANGED ACFT IN OAK; FLEW TO SMF; AND CONTINUED WITH OUR DAY. UPON CONFIGN IN OAK; WE HAD NO DIFFICULTIES WITH THE GEAR OR ITS INDICATIONS. WHEN THE LNDG GEAR WAS EXTENDED; WE RECEIVED 3 GREEN LIGHTS AND NO RED UNSAFE LIGHTS. IF THIS SIT HAPPENED AGAIN TODAY; I WOULD DO THE SAME ACTIONS. BECAUSE OF THE MIXED WX AND TERRAIN IN ONT; WE DID THE SAFEST THING BY STAYING ON THE SID AND KEEPING OUR SITUATIONAL AWARENESS. THE CAPT AND I BACKED UP OUR ACTIONS WITH THE QRH; AND COMMUNICATED EFFECTIVELY WITH COMPANY. WE KEPT THE CABIN CREW AND PAX INFORMED WHEN WE REALIZED THAT WE'D BE DIVERTING TO OAK.