What was found
AT 31,000 FT, NR 3 ENGINE AIR VALVE LIGHT CAME ON, THEN OIL 3 LOW PRESSURE LIGHT CAME ON, VERIFIED WITH OIL PRESSURE GAUGE IN RED, CREW SHUT DOWN ENGINE. DIVERTED TO STL. MAINTENANCE INVESTIGATED DISCREPANCY, NO LEAK NOTED AND NO FAULT ON ECU. PERFORMED GROUND RUNS PER AMM 71-00-00, PERFORMED OVERSPEED TEST, REPLACED ECU. PERFORMED GROUNDS RUN, OPS CHECK GOOD.