As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU DET INOP LIGHT CAME ON DURING PUSHBACK AND REMAINS ILLUMINATED. GROUNDED - YES. PERFORMED FIM 26-10 TASK 801 AND FAULTED LOWER APU DETECTOR. REPLACED LOWER APU OVERHEAT DETECTOR ELEMENT PER AMM 26-15-01. PERFORMED GOOD OPS TEST OF APU FIRE DETECTION PER AMM 26-15-00-710-801.