As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE ON THE RAMP, APU SHUT DOWN UNCOMMANDED. THE APU WAS DEFERRED IAW MEL. TROUBLE SHOT APU FIM & REPLACED FUEL CONTROL UNIT PN 4504616B, BUT IT DIDN'T FIX THE PROBLEM. INSPECTED APU CHIP DETECTOR & FOUND A MOUND OF METAL ON IT. PLACED AN ORDER FOR APU UNIT & LT THE APU ON MEL.