As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ETOPS SIGNIFICANT SYSTEM: DURING ENGINE RUN UP GENERATOR NR 1 SOURCE OFF LIGHT ILLUMINATES. FOR T/S PURPOSES SWAPPED GCU BETWEEN APU AND NR 1 GEN GCU OPS CKS NORMAL FOR APU AND GEN GCU APU GCU P/N 76215F/ S/N 2792 GEN GCU P/N 762185F S/N 2731 IAW 737-800 FIM 24-21 TASK 801.