What was found
THE ENTERED COST INDEX DELETED ITSELF AT CRUISE, AS A RESULT, ALL SPEEDS AND ALTITUDE HOLD, NAVIGATION WA S NOT AFFECTED, ONCE THE COST INDEX RE-ENTERED, ALL OPERATION RETURNED TO NORMAL,. GROUNDED - YES. PERFORMED BOTH FMCS BITE FOUND NO INFLIGHT FAULTS & SENSOR FAULTS, ALSO PERFORMED FMCS OPS TEST NORMAL B737-900 FIM34-61 TASK 801 & 34-61-00-710-801. NO MX HISTORY 90 DAYS.