As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PIT - FLT 348 - AFTER TAKEOFF, THE APU FIRE LIGHT ILLUMINATED. THE CAPTAIN DISCHARGED THE BOTTLES AND THE LIGHT EXTINGUISHED. FLIGHT RETURNED TO PIT. MAINTENANCE FOUND THE APU FIRE DETECTION WIRE CHAFED THROUGH. THE DETECTION ELEMENT, FIRE BOTTLE AND FIRE WIRE WAS REPLACED. OPERATIONS CHECKED GOOD. (W)