What was found
PDX FLIGHT NR 2900: AFTER TAKEOFF, RT MAIN LANDING GEAR DOOR AND GEAR UNSAFE LIGHT ILLUMINATED AND BEGAN TO CYCLE ON AND OFF. PERFORMED EMERGENCY CHECKLIST AND LOWERED GEAR, PUT GEAR BACK UP AND ONLY GEAR DOOR LIGHT ILLUMINATED AND BEGAN TO CYCLE ON AND OFF. AIRCRAFT RETURNED TO FIELD. MAINTENANCE ADJUSTED RT MAIN DOOR PROX SENSOR IAW Q400 TASK 32-12-01-830-801. C/W OPS TEST OF LANDING GEAR EXTENSION/RETRACTION SYSTEM IAW Q400 AMM 32-12-01-830-801. NO PARTS REPLACED. AIRCRAFT RETURNED TO SERVICE.