As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU DET INOPERATIVE ANNUNCIATOR ILLUMINATED IN FLIGHT. ACFT GROUNDED. PERFORMED ENGINE/APU FIRE DETECTION CONTROL MODULE BITE IAW FIM TASK 26-10, TASK 801. FOUND TAILPIPE DETECTOR RESISTANCE READING OUT OF TOLERANCE. R & R APU TAILPIPE OVERHEAT DETECTOR ASSY IAW AMM 26-15-01/401. APU FIRE DETECTION OPS TESTED NORMAL.