As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DFW - CREW REPORTED FIRE LOOP B INDICATION DURING FLIGHT. FLIGHT RETURNED DFW LANDED WITHOUT INCIDENT. AIRCRAFT REMOVED FROM SERVICE. RESET FIRE DETECTOR LOOP B AUXILIARY POWER UNIT CIRCUIT BREAKER AND REPLACED AUXILIARY POWER UNIT FIRE CONTROL AS A PRECAUTION. SYSTEM GROUND CHECKED NORMAL OPERATION.