As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CRUISE FL 380, MASTER CAUTION ELEC, SOURCE LT + DRIVE LT ENG #1 ILLUMINATED. QRH DISCONNECTED IDG 1 AND COVERED BUS W/ APU. GROUNDED - YES. REPLACED IDG AIR/OIL COOLER REF B737-900 AMM 24-11-21-000-801. REMOVED AND REPLACED #1 ENG IDG REF AMM 24-11-11. PERFORMED ENGINE TEST 3A IDLE LEAK CHECK AND IDG OPERATIONAL TEST PER AMM 24-11-00-700-802. LEAK AND OPS CHECK GOOD NO FURTHER DEFECTS NOTED.