What was found
(CAN) LANDING GEAR WOULD NOT RETRACT & CONFIGURATION HORN SOUNDED AT TAKEOFF. A/C RETURNED AS PER QRH PROCEDURE. MAIN R/H LANDING GEAR OLEO SERVICED. ON SUBSEQUENT FLIGHT, L/R GEAR SHOWED RED. GEAR SELECTED DOWN 3 GREEN. L/G ACCESSORY UNIT CHANGED. A/C TEST FLOWN WITH SAME PROBLEM. TROUBLESHOOTING CARRIED OUT. FOUND PIN FOR THE UPLOCK SENSOR PUSHED BACK AT JUNCTION BOX. PIN REINSTALLED AND SYSTEM TESTED AS PER MM FOUND TO BE SERVICEABLE. NO FURTHER PROBLEM DURING SUBSEQUENT FLIGHT.