As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU DOOR STATUS MSG SAMC TEXT: MESSAGE OCCURED AFTER SHUTDOWN. CREW CONTACTED SAM, FAULT WENT OUT WHEN APU WAS RESTARTED BUT RETURNED WHEN APU WAS TURNED OFF. FOUND WHEN STARTING APU VOLTAGE DROPS TO 12V INITIAL AND CONTINUALLY CHARGES AT 37AMPS. REPLACED APU BATTERY IAW AMM, APU START CHECKS OKAY.