As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT CRUISE WITH APU RUNNING APU DET INOP LIGHT ILLUMINATED. PERFORMED FIM 26 10 00 810 801. FOUND APU TAILPIPE DETECTOR M1925 TO BE AT FAULT. FAULT CONFIRMED. REMOVED AND REPLACED M1925 PER MM 26 15 01 400 802. VERIFIED PART NUMBER SEAMC. OPS TESTED PER MM 26 15 00 710 801. TESTS OK.