As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON TAXI OUT, RECEIVED AN APU FIRE WARNING. MAINTENANCE INSPECTED APU COMPARTMENT AND NO EVIDENCE OF FIRE DAMAGE OR OVERHEAT CONDITION WERE NOTED. INSPECTED APU OVERHEAT DETECTION SYSTEM AND FOUND APU TAILPIPE OVERHEAT DETECTOR LOOP M717 SHORTED TO GROUND. REPLACED APU OVERHEAT DETECTOR LOOP M717.