What was found
(CAN) THE LANDING GEAR FAILED TO EXTEND WHEN GEAR LEVER WAS SELECTED DOWN. A MISSED APPROACH WAS CARRIED OUT. AT A SAFE ALTITUDE (3000 FT) THE QRH CHECKLIST WAS INITIATED. THE ALTERNATE LANDING GEAR SYS WAS USED. LANDING GEAR EXTENDED (3 GREENS) AND A NORMAL APPROACH AND LANDING WAS EXECUTED. LINE MX INVESTIGATION CONFIRMED FAILURE OF THE LANDING GEAR SELECTOR VALVE. VALVE ASSY REPLACED AND LANDING GEAR FUNCTIONALLY TESTED IAW THE ACFT`S MM. ACFT RELEASED BACK TO SERVICE WITHOUT FURTHER INCIDENT.