As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER APU START, APU ELEC SHOWS NO VOLTS OR FREQ. BLUE APU AVAIL LIGHT IS ON. GROUNDED - YES. COULD NOT DUPLICATE. SWAPPED APU GCU AND #1 ENGINE GCU. BOTH SYSTEMS OP'ED NORMAL. RETURNED GCU'C TO ORIGINAL POSITIONS. BOTH #1 ENGINE GENERATOR AND APU GENERATOR OP NORMAL REF B737-900 AMM 24-21-00.