As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APPROXIMATELY 3 HOURS INTO FLIGHT, THE MAIN DECK FIRE LIGHT AND WARNING BELL ACTIVATED SIMULTANEOUSLY. DURING QRH PROCEDURE LIGHT EXTINGUSHED ON ITS OWN, NO VISUAL EVIDENCE OF FIRE. ACCOMPLISHED BOW AND FOUND CORROSION ON YW8A04 DIODE CARD, REPLACED THE CARD.