As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
MASTER CAUTION, OVERHEAT, DETECTION LIGHT FLASHING INTERMITTENTLY WITH APU DET INOPERATIVE LIGHT. ACFT GROUNDED. APU FIRE DETECTION TEST NORMAL NUMEROUS TIMES, NO FAULT ON ENGINE & APU FIRE BOX. ACCOMPLISHED APU LOOP RESISTANCE CHECKS REF AMM 26-15-00 WITHIN LIMITS.