What was found
AIRCRAFT WAS GROUNDED: LANDING GEAR HORN CAME ON AND ALL THREE GEAR LIGHTS WENT TO RED . WE WERE AT 10,000FT SLOWING FROM 320 KNOTS TO 250 KNOTS. THRUST LEVERS WERE AT IDLE. GEAR HORN STAYED ON FOR SEVERAL MINUTES UNTIL WE FINALLY LOWERED THE LANDING GEAR AND IT TURNED OFF AND THE GEAR LIGHTS TURNED GREEN. LANDING GEAR EXTENDRF NORMALLY AND A NORMAL LANDING WAS MADE. MAINTENANCE REPLACED PROXIMITY SWITCH ELECTRONICS UNIT (PSEU) PER WORK CARD 08L00032003.