A B737-300 ON TKOF CLB; WHEN GEAR RETRACTED HAD L MAIN GEAR RED LIGHT. RECYCLING THE GEAR DID NOT RESOLVE THE PROBLEM. FLC EXTENDED GEAR; GOT THREE GREEN; RETURNED TO DEP ARPT.

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WX CONDITIONS WERE DAYLIGHT; VMC. I WAS THE PF; FO WAS THE PNF. AFTER A NORMAL TKOF I CALLED FOR THE GEAR TO BE RETRACTED PER SOP. ON RETRACTION; THE 3 GREEN LIGHTS EXTINGUISHED AND ALL 3 RED LIGHTS ILLUMINATED PROPERLY. THE L MAIN GEAR RED LIGHT NEVER EXTINGUISHED AFTER RETRACTING. WHEN THE GEAR HANDLE WAS MOVED TO THE OFF POSITION PER THE AFTER TKOF CHKLIST; THE L MAIN GEAR GREEN LIGHT ILLUMINATED WITH THE L MAIN GEAR RED LIGHT ALSO ILLUMINATED. WE REQUESTED AND RECEIVED CLRNC TO LEVEL OFF AT 6000 FT ON A RADAR VECTOR AND A SPD OF 210 KTS. I TRANSFERRED CTL OF THE ACFT TO FO AS WELL AS ATC COMMUNICATIONS. THE FLT ATTENDANTS WERE ADVISED NOT TO BEGIN CABIN SVC AND TO WAIT FOR AN UPDATE. A PHONE PATCH WITH DISPATCH AND MAINT CTL WAS INITIATED AND AFTER DISCUSSION WITH THEM WE AGAIN ATTEMPTED TO RETRACT THE GEAR. THE L MAIN GREEN LIGHT AGAIN EXTINGUISHED; HOWEVER THE RED LIGHT AGAIN REMAINED ON. AFTER MORE CONSULTATION WITH MAINT CTL WE ATTEMPTED TO AND SUCCESSFULLY LOWERED THE GEAR. ALL THREE GEAR PROPERLY CAME DOWN AND ALL THREE RED LIGHTS EXTINGUISHED. FLT ATTENDANTS AND PAX WERE ADVISED TO EXPECT A NORMAL APCH AND LNDG IN ZZZ (NO CABIN PREP OR ADVISORY ISSUED). WE MADE THE DECISION TO RETURN TO ZZZ AND I TOOK CTL OF THE ACFT. WE RECEIVED RADAR VECTORS TO LAND UNEVENTFULLY. AN INSTRUMENT APCH WAS CONDUCTED DOWN TO APPROX 400 FT; FOLLOWED BY A NORMAL LNDG AND TAXI TO THE GATE. AFTER THE ARR WE WERE ABLE TO VISUALLY DETERMINE THAT THE L WING BODY FAIRING BETWEEN THE L INBOARD FLAP AND THE FUSELAGE HAD BECOME LOOSE AND WAS HANGING DOWN. ITS POSITION WOULD HAVE BEEN IN THE PATH OF THE L MAIN GEAR WHILE RETRACTING. IT WAS ALSO DISCOVERED THAT THE L MAIN STRUT DOOR WAS CRACKED FROM THE LEADING TO THE TRAILING EDGE.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT SINCE THEY HAD THREE GREEN LIGHTS WITH THE GEAR DOWN AND LOCKED IT WAS DETERMINED TO RETURN TO THE DEP FIELD. NO EMER WAS DECLARED AND THE LNDG WAS NORMAL. ON INSPECTION OF THE L MAIN GEAR IT WAS DISCOVERED THE L WING BODY FAIRING WAS LOOSE AND HANGING DOWN INTO THE WHEEL WELL GEAR CUTOUT. THE FAIRING WAS MISSING MANY SCREWS AND WAS NOT SECURE. THE LNDG GEAR STRUT DOOR WAS ALSO FOUND CRACKED. THE RPTR STATED A WALK AROUND INSPECTION WOULD NOT HAVE REVEALED THIS UNSECURED FAIRING AS THE TRAILING EDGE FLAPS WERE RETRACTED AND COVERED THE FAIRING.