What was found
DURING CLIMB OUT, AT APPROXIMATELY 19,000 FEET, CAPTAIN REPORTED THAT DUCT LEAK WARNING WAS RECEIVED WITH CORRESPONDING LEFT DUCT LEAK LIGHT ILLUMINATING. CAPTAIN RAN CHECKLIST. WITH APU BLEED CLOSED THE WARNING INDICATION WENT OUT BRIEFLY THEN RETURNED. WHEN LEFT BLEED WAS CLOSED INDICATION WENT OUT, BRIEFLY CAME BACK ON, THEN WENT AWAY AGAIN. DECISION WAS MADE TO RETURN TO AIRPORT. EMERGENCY WAS DECLARED. UPON CONTINUATION OF DUCT LEAK CHECKLIST, CAPTAIN FEATHERED THE LEFT ENGINE AND INDICATION LIGHT EXTINGUISHED, ENGINE WAS NOT SHUTDOWN. SINGLE ENGINE CHECKLIST WAS PERFORMED, AIRCRAFT LANDED SAFELY. REPAIRED WIRING TO SENSOR AT JUNCTION TO APU BLEED DUCT. OPERATION CHECKS OK. A/C OK FOR SERVICE.