As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
BOTH MASTER FIRE WARNING LIGHTS ILLUMINATE INTERMITTENTLY. ALL FIRE WARNING LIGHTS OFF PRIOR TO TAKEOFF. ONCE AIRBORNE APU FIRE LIGHT CAME ON STEADY. MASTER FIRE WARNING LIGHTS STARTED FLICKERING. REPLACED APU FIRE BOTTLE. FOUND CORROSION ON PLUG D-4858P FOR R421 BLOCKING RECTIFIER. CLEANED SAME, ALSO REPLACED AURAL WARNING BOX. FIRE SYSTEM TESTS OK. RAN APU, NORMAL OPERATION.