As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON TAXI OUT HAD 3 DIFFERENT ACFT SAY WE HAD FLAMES COMING OUT APU EXHAUST. PUSHED APU FIRE SWITCH & DISCHARGED THE FIRE AGENT. FIRE FROM TAILPIPE WENT OUT. RETURNED TO GATE WITH EMER EQUIPMENT FOLLOWING. INSPECTED APU IAW AMM 49-00-00-200-001-A. INSPECTION COMPLIED WITH, NO DEFECTS NOTED.