What was found
APU FAILED TO START. UNABLE TO UNLATCH IAW FIM DUE TO LACK OF AIRSTART AVAILABILITY. IT S/B NOTED THAT CHECK VALVE REPLACED ON 9TH FEB & HAS CAUSED THIS PROBLEM SINCE FITTMENT. PLACED BLEED OFF & STARTED APU ELECTRICALLY BUT WHEN BLEED VALVE OPENS GET APU BLEED AIR STATUS MSG. APU BLEED AIR VALVE LOCKED IN CLOSED POSITION IAW MEL. REPLACED CHECK VALVE, RESET APUC, NO HELP. SUSPECT TOTAL PRESS SENSOR OR DIFF PRESS SENSOR. BOTH PARTS NIS LHR. INSTALLED ORIGINAL CHECK VALVE. REPLACED APUC, TOTAL PRESSURE SENSOR & DIFF PRESSURE SENSOR. NEED TO VERIFY FIX BY STARTING ELECTRICALLY & WITH AIR. ACCOMPLISHED APU AIR START & OPS OF BLEED WITH ENG AT IDLE & ELECTRICAL START. NO FAULTS NOTED.