What was found
AIR TURNBACK, NO EMERG DECLARED. ALT DISAGREE EICAS MESSAGE PASSING 6000'MSL, ALT DIFFERENCE OF 320' BETWEEN CAPT & F/O INST,COMPLETED AIRSPEED UNRELIABLE PROCEDURE(W/ALTN AIR DATA SELECTED(CAPT SIDE), CAPT & F/O A/S & ALT COMPARISON FL 280 ALT SPLIT 300' A/S SPLIT 6 KTS.PERFORMED AOA SENSOR TEST BY ARINC 429 ANALYZER PER MM 34-12-03-401. FOUND OUT OF LIMIT AT LEFT VANE POSITION SET -10. TESTER SHOWS 349.233. ALL OTHER POSITION W/IN LIMIT. REPLACED LH AOA SENSOR & TEST OK PER MM 34-12-03-401.