As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
B-18755 AIRCRAFT AIR TURN BACK BECAUSE NO.4 BTB COULD NOT CLOSE WITH GEN OPEN LT ILLUM.GEN ABNORMAL CSD DISCONNECTED.C/A:1.GCU SWAPPED WITH #1,FOUND #4 FAILED. 2.SHOP AND QA INSPECTED THE GCU,AND REPORTED THAT THERE WERE MOISTURE IN THE GCU. 3.THE REST OF GCU S WERE ALSO REMOVED FOR BENCH CHECK. 4.E.O.N WAS PERFORMED TO RE-SEAL THE FLOOR PANELS AND CLEAN THE DRAINS,NO MORE LEAK WAS FOUND.