What was found
DURING TAXI AFTER ENG STARTS, NO PACK PRESSURE OBSERVED ON LT SIDE WITH ISOLATION SWITCHES CLOSED. ACFT RETURNED TO THE GATE (BTB) AND MTX GROUNDED A/C . R & R LT ENG PRSOV IAW AMM 36-11-09. PERFORMED VISUAL INSP TO LT ENG PRSOV CONNECTOR D22 IAW WDM 36-11-51. NO DISCREPANCIES NOTED. C/W L ENG PNEUMATIC SYST HEALTH CHECK, OPS CHKD IAW AMM 36-00-21/501 SYSTEM GOOD. PER T/S PURPOSE SWAPPED AIR SUPPLY CARD BETWEEN LT AND RT ENG IAW AMM 36-11-12/401. INSTALLATION TEST NORMAL. ACCOMPLISHED AIR SUPPLY SYS OPS CHKS ON LT & RT SIDES PER AMM 36-11-00 AND 71-00-00. ALL CHKS NORMAL. ISOLATION & PRESSURES NORMAL. NO MESSAGES. ACFT REMAINS ETOPS 180. ACFT RETURNED TO SERVICE. GE, CF680C2B6