As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMBOUT, THE APU FIRE WARNING LIGHT ILLUMINATED. CREW PERFORMED COM PROCEDURES AND DISCHARGED THE FIRE EXTINGUISHING AGENT FROM THE NR 1 FIRE BOTTLE. FLIGHT RETURNED TO A NORMAL LANDING AT MSP. MAINTENANCE FOUND NO EVIDENCE OF FIRE OR SMOKE. THE APU FIRE DETECTION SYSTEM WAS SHORTING. REPLACED THE APU FIRE DETECTION SYSTEM AND THE NR 1 FIRE BOTTLE. AIRCRAFT RETURNED TO SERVICE.