FLC RETURNED TO LAND AFTER NOT BEING ABLE TO RETRACT LNDG GEAR. MAIN GEAR PINS WERE LEFT IN FOR TKOF BY MISTAKE.

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WE WERE SCHEDULED TO DEPART CVG FOR AUP AT XA10. OUR ACFT WAS HAVING THE MAIN LNDG GEAR BAY OVERHEAT SENSOR REPLACED AND WE DEPARTED 63 MINS LATE. AFTER MAINT WAS COMPLETED I CONDUCTED MY WALKAROUND AND NOTICED NO IRREGULARITIES. THERE WERE 3 GEAR PIN FLAGS ON THE HOLDERS ON THE FLT DECK BULKHEAD. START AND TAXI WERE NORMAL. ON TKOF WHEN GEAR WERE SELECTED UP THE NOSE GEAR RETRACTED AND THE MAINS REMAINED DOWN. WE SELECTED THE GEAR HANDLE DOWN AND GOT 3 GREEN INDICATORS. WE SWITCHED TO DEP AND INFORMED THEM OF THE PROB (AFTER INFORMING TWR AND TELLING THEM THAT WE NEEDED TO REMAIN IN THE LCL AREA). WHILE BEING VECTORED TO BURN DOWN TO MAX LNDG WT WE INFORMED THE PAX AND COMPANY. UPON REACHING MAX LNDG WT WE MADE AN UNEVENTFUL APCH AND LNDG. AFTER LNDG WE DISCOVERED THAT THE ACFT HAD FLOWN A REVENUE FLT FROM GSP TO CVG WITH THE MAIN GEAR PINNED BECAUSE THE MAIN LNDG GEAR BAY OVERHEAT SYS WOULD NOT TEST. THE FLAGS WERE REMOVED FROM THE GEAR PINS AND PLACED IN THE HOLDERS IN THE COCKPIT. THERE WAS NO MAINT WRITE-UP TO INDICATE THE ACFT HAD BEEN FLOWN WITH THE GEAR PINNED. THE MECHS WHO REPLACED THE WARNING ELEMENT WERE NOT AWARE OF THE GEAR PINS BEING INSTALLED AND NEITHER WERE WE. WITHOUT THE FLAGS ON THE GEAR PINS; IT IS NEARLY IMPOSSIBLE TO TELL THAT THEY ARE INSTALLED. OBVIOUSLY; THIS INCIDENT WAS COMPLETELY AVOIDABLE HAD THE PROPER MAINT PROCS BEEN FOLLOWED. I SHARE A PORTION OF THE BLAME FOR NOT LOOKING CLOSELY DURING MY PREFLT.