What was found
FUEL QUANTITY INITIALLY 40.8 TOTAL 8.64 / 23.5 / 8.64. LESS THAN 5 MIN AFTER ENGINE START QTY WAS 7.68 / 22.6 / 8.6 (#1 QTY DROPPED NEARLY 1000 POUNDS). NO OTHER ABNORMAL INDICATIONS. ALL 6 PUMPS ON. NOTE "TOTAL" FUEL ALSO DROPPED (MIRRORED THE LOSS #1 QTY INDICATION). GROUNDED - YES. ACCOMPLISHED FQIS SYSTEM TEST #1 TANK COMP 59.03 406.9 TOTAL. REMOVED AND REPLACED #1 COMP AND 1-5 TANK UNITS IAW B737-800 AMM 28-41-21/401. REMOVED AND REPLACED FUEL PROBES 9-12 REF AMM 28-41-21/401. REMOVED AND REPLACED FUEL PROBES 7 AND 6 IAW AMM 28-41-21/401. REMOVED AND REPLACED FUEL PROBE #8 REF AMM 28-41-21. FQIS OPERATIONAL CHECK COMPLETE AND #1 MAIN FUEL TANK LEAK CHECK COMPLETE. OPS CHECK GOOD.