As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SHIP 9575 -ATB/DE- AS THE AIRCRAFT CLIMBED OUT ON DEPARTURE, THE APU FIRE WARNING (LEVEL 2) ACCOMPANIED BY THE MASTER CAUTION, ALL ILLUMINATED. THE CREW DECLARED AN EMERGENCY AND RETURNED THE AIRCRAFT BACK TO DTW. THE LANDING WAS UNEVENTFUL AND ARFF SCANNED THE AIRCRAFT WITH NO HOTSPOTS LOCATED. NORMAL TAXI TO THE GATE. CORRECTIVE ACTION: MAINTENANCE REPLACED THE APU FIRE DETECTION CONTROL UNIT. OPERATIONAL CHECK WAS GOOD AND THE AIRCRAFT RETURNED TO SERVICE. (LP #5892292)