As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT RETURNED TO FIELD DUE TO AN APU FIRE WARNING INDICATION. AIRCRAFT EXPERIENCED APU FIRE WARNING WITH APU OFF DURING CLIMB. CREW DISCHARGED APU FIRE BOTTLE AND RETURNED TO THE FIELD. MAINTENANCE INSPECTED APU OVERHEAT DETECTION SYSTEM AND FOUND A HIGH RESISTANCE SHORT TO GROUND AT CONNECTOR D1000, PINS 20 & 26. REPLACED APU FIRE DETECTION LOOP M334.