What was found
AT FL 350 WE TURNED ON WING ANTI-ICE. WE THEN RECEIVED A LEVEL 3 AIR MANF 1 FAIL. ACCOMPLISHED A DECAY CHECK, IAW AMM 36-00-01-5-0. SYSTEM PASSED DECAY CHECK. INSPECTED PNEUMATIC FAULT MANIFOLD INDICATOR AND FOUND A FAULT BALL FOR THE LT MANIFOLD FAILURE DETECTION SWITCH (SWITCH S1-654). REMOVED AND REPLACED LT MANIFOLD FAILURE DETECTION SWITCH MGF P/N 83165-1 S/N 3253, IAW AMM 36-23-03-4. LEAK CHECK AND OPERATIONS CHECK GOOD. ALSO ACCOMPLISHED AN AIR SYTEM TEST AND RTS OF PSC 1. RAN NR 1 ENGINE, IAW JRH AND OPERATED WING ANTI ICE. NO FAULTS NOTED AT THIS TIME. A/C OK FOR SERVICE.