As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) SHORTLY AFTER TAKEOFF, APU FIRE WARNING LIGHT CAME ON AND BELL SOUNDED. APU SHUT DOWN. FIRE DRILL COMPLETED. FIRE LIGHT REMAINED ON AND FIRE BELL SOUNDED INTERMITTENTLY. AIRCRAFT RETURNED. REPLACED TAIL PIPE AND EXHAUST DUCT FIRE DETECTION LOOPS AS RESISTANCE WAS TOO LOW.