As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR TURNBACK. APU FIRE WARNING LIGHT ILLUMINATED AND BELL SOUNDED. APU FIRE EXTINGUISHER DISCHARGED. APU WAS NOT OPERATING. INSPECTED APU COMPARTMENT, NO EVIDENCE OF FIRE NOTED. FOUND APU TAIL PIPE OVERHEAT SENSOR FAULTY. DEFERRED APU PER MEL.