As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE APU FAULT LIGHT ILLUMINATED & THE APU SHUTDOWN. THE APU WAS RESTARTED, THEN SHUTDOWN AGAIN. THE FLIGHT WAS DIVERTED. THE APU ELECTRONIC CONTROL UNIT WAS REPLACED IAW THE AMM. ELECTRONIC CONTROL UNIT’S POWER SUPPLY FILTER FL4 FAILED.