What was found
(CAN) DURING INITIAL CLIMBOUT, THE FLIGHT CREW SELECTED LANDING GEAR UP AND OBSERVED 3 RED AND 3 GREEN LANDING GEAR INDICATION LIGHTS ILLUMINATE SIMULTANEOUSLY AND THE GEAR DID NOT RETRACT. AFTER THE APPROPRIATE CHECKLIST WAS CARRIED OUT, THE FLIGHT CREW ELECTED TO EXECUTE AN AIR TURN BACK. AFTER A SAFE AND UNEVENTFUL LANDING THE ACFT RETURNED TO GATE WHERE IT WAS MET BY MAINTENANCE. MAINTENANCE CARRIED OUT ACFT FIM TASK 32-30, TASK 815 AND DISCOVERED THE LANDING GEAR MANUAL EXTENSION DOOR SWITCH, PN MS25011-4 TO BE AT FAULT PREVENTING THE LANDING GEAR TO RETRACT WHEN SELECTED. THE SWITCH WAS SUCCESSFULLY REPLACED IAW AMM 32-34-11 AND THE ACFT WAS RETURNED TO SERVICE.