As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
"APU DET INOP" LIGHT IS ILLUMINATED. ACFT GROUNDED. PERFORMED APU FIRE DETECTOR CONTROL MODULE BITE IAW FIM 26-10, TASK 801. FAULTED APU TAILPIPE DETECTOR. RESISTANCE CHECK ON TAILPIPE DETECTOR IS 64 MEG OHMS. REPLACED APU DETECTOR WITH SERVICABLE ONE. ACCOM INST TEST PASS, REF MM 26-15-00-710-801.