What was found
ON THE GROUND AT THE GATE. APU ON. NO EXTERNAL AIR. ALL BLEEDS ON, PACKS AUTO, ISO VALVE OPEN. DUCT PRESSURE WENT TO ZERO. DUAL LIGHT FLICKERED AND ANC AIR COND ILLUMINATED. THIS HAPPENED 3 TIMES. GROUNDED - YES. INTERROGATED AIR CONDITIONING SYSTEM. FOUND ACTIVE FAULT 21-55341. PERFORMED AIR COND IBIT-R SYSTEM TEST. TEST PASSED. PERFORMED IBIT FLOW SENSOR TEST-R. TEST FAILED. MAINT MSG 21-55061- RIGHT AIR COND PACK FLOW SENSOR IS OUT OF RANGE. R/R RH PACK FLOW SENSOR IAW B737-9 AMM 21-61-24-000-801. OPS CHECK IAW AMM 21-00-00-730-802, GOOD.