As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING OUTBOUND TAXI, THE APU FIRE WARNING LIGHT ILLUMINATED. THE FIRE BOTTLE WAS DISCHARGED, TWO MINUTES LATER DURING RETURN TO GATE, THE FIRE WARNING LIGHT ILLUMINATED AGAIN. APU WAS INSPECTED WITH NO EVIDENCE OF FIRE FOUND AND APU WAS PLACED ON MEL.