As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WITH THE APU SUPPLYING ELECTRIC POWER WITH THE APU GENERATOR THE VOLTS AND FREQ DROPPED TO ZERO THEN RETURNED TO NORMAL. GROUNDED - YES. PERFORMED APU GCU BITE REF B737-800 FIM 24-21 TASK 801, NO FAULTS FOUND. ALSO PERFORMED APU BITE WITH CDU REF B737-800 AMM 79-61-00-700-801, NO FAULTS FOUND. OPERATED APU GENERATOR REF AMM 49-11-00-710-801, OPS CHECK GOOD NO DEFECTS NOTED.