As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING TAXI - AIRCRAFT BLOCKING IN, THE APU FIRE WARNING CAME ON. FIRE HANDLE WAS PULLED BUT BOTTLE NOT DISCHARGED. POST EVENT INSPECTION FOUND NO INDICATION OF A FIRE HAVING OCCURRED IN APU COMPARTMENT. ALSO DISCOVERED THAT APU FIRE BOTTLE HAD BEEN DISCHARGED "AUTOMATICALLY". SYSTEM WAS SUBSEQUENTLY DEFERRED. ON 09/23/2021, THE APU FIRE BOTTLE AND SQUIB WERE REPLACED WITH NO FURTHER ABNORMAL INDICATIONS NOTED.