What was found
SEA - ON FLIGHT NUMBER 2340, DE-ICE CAUTION LIGHT ILLUMINATED WHEN DE-ICE BOOT MODE SELECTED IN AUTO (FAST OR SLOW). RIGHT HAND INBOARD GREEN BOOT ON TAIL FAILS TO ILLUMINATE. RAN CHECKLIST AND NO DROP IN ISO WHEN USING A MANUAL SELECTION OF THE BOOTS, THE GREEN TAIL LIGHT STILL DOES NOT ILLUMINATE, BUT THERE IS NO DE-ICE PRESS CAUTION LIGHT. THE NR 5 DUAL DISTRIBUTION VALVE OPERATED INTERMITTENTLY. REMOVED AND REPLACED NR 5 DUAL DISTRIBUTION VALVE. OPERATIONAL CHECK GOOD. AIRCRAFT RETURNED TO SERVICE.