What was found
FUEL TANK NR 1 INDICATED 1500 LBS LESS FUEL THAN FUEL TANK NR 2. WHILE RUNNING THE FUEL IMBALANCE AND ENGINE FUEL LEAK CHECKLISTS, THE FUEL TANK NR 1 INDICATOR SUDDENLY WENT BACK UP 1500 LBS TO ITS ORIGINAL VALUE. SUSPECT A FUEL GAUGE OR SENSOR ERROR. GROUNDED - YES. BITE CHECKED FQIS REF FIM 28-41 TASK 801. 2 EA FLIGHT FLTS 28-41205 TANK 2, 28-41105 TANK 1. ACCOMPLISHED OPERATIONAL TEST OF THE FUEL QUANTITY INDICATION SYSTEM IAW AMM 28-41-00-710-801. GROUND TEST PASS. ACCOMPLISHED RESISTANCE AND CAPACITANCE CHECK OF THE NR 1 MAIN TANK IAW AMM 28-41-21-710-801. VALUES OF NR 1 MAIN WITHIN LIMITS. ACCOMP RESISTANCE & CAPACITANCE CHECK OF NR 2 MAIN TANK REF AMM 28-41-21-710-801. ALL VALUES ARE WITHIN LIMITS. REMOVED & REPLACED FQIS PROCESSOR UNIT, PRECAUTIONARY, REF AMM 28-41-81-400-801. ACCOMP FQIS OPERATIONAL TEST REF AMM 28-41-00-710-801. OPS CK GOOD.