As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE APU OIL COOLER WAS REPLACED AND THE AREA CLEANED FOR LEAK CHECK. A MAINTENANCE STAFF WAS IN THE COCKPIT STARTING THE APU WITH HIS MAINTENANCE CREW AT THE APU AREA WITH APU DOORS OPEN TO PERFORM THE LEAK CHECK. UPON APU START-UP, FLAMES WERE OBSERVED IN THE APU AREA. THE MAINTENANCE STAFF IN THE COCKPIT IMMEDIATELY PULLED APU FIRE HANDLE TO CUT OFF THE FUEL SUPPLY, AND THE FLAMES SELF-EXTINGUISHED AFTER A SHORT WHILE, IN LESS THAN A MINUTE.