What was found
AFTER T/O, L/G WAS SELECTED UP, BUT NLG DID NOT RETRACT (MAINS OK). NLG AMBER CAUTION LIGHT WAS ILLUMINATED, CREW SELECTED THE GEAR BACK DOWN AND NLG THEN INDICATED A RED UNSAFE LIGHT. CREW CARRIED OUT THEIR EMERGENCY CHECK LIST, ALT EXTENSION ON BOTH GEARS, WITH ALL GEARS THE INDICATING DOWN AND LOCKED. A/C THEN MADE A SAFE LANDING. INITIAL CHECKS OFF PSEU SHOWED NO ABNORMAL FAULTS, WITH THE NLG PROX SENSORS IMPEDANCE AND RESISTANCE CHECKS OK. A/C WAS TAKEN TO MX HANGAR FOR JACKING AND LANDING GEAR RETRACTIONS TESTS/TROUBLE SHOOTING. FAULT WAS REPRODUCED WITH A/C ON JACKS, WITH FAULT BEING TRACED TO THE NLG DOOR SOLENOID SEQUENCE VALVE. NLG DOOR SOLENOID SEQUENCE VALVE WAS REPLACED.