What was found
AFTER TAKEOFF LANDING GEAR UNSAFE LIGHT CAME ON, THE CREW CYCLED GEAR LIGHT WENT OUT. ONE HR INTO FLIGHT DOOR NOT LATCH LIGHT CAME ON THEN WENT OUT, THE LANDING GEAR UNSAFE LIGHT REMAINED ON DURING FLIGHT. UPON GEAR EXTENSION, A LOUD BANG WAS HEARD, GOT (3) GREEN LIGHTS. THE CREW LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE INSPECTED LANDING GEARS AND FOUND LEFT MAIN LANDING GEAR RETRACT PIN BROKEN. THEY REPLACED THE LEFT MAIN LANDING GEAR STRUT ASSY. OPERATIONAL CHECK GOOD IAW UAL MAINTENANCE MANUAL 32-11-03. AIRCRAFT WAS RETURNED TO SERVICE.