As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU FIELD OFF LT IS ON. WHEN GND PWR WAS DISCONNECTED PLANE WENT DARK. HMG VERIFIED TO OP NORMAL AS PER MEL. NEED TO REPLACE APU GENERATOR PER MM. NEED LEAK CHECK AND SYS TEST PER MM. REPLACED GENERATOR PER MM. OPS CHECK OF APU GEN NORMAL. REPLACED GCU PER MM.