As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SMOKE OBSERVED AT BASE OF HORIZONTAL STAB WITH APU RUNNING AT GATE. FOUND APU DOOR ACTUATOR AS SOURCE OF SMOKE. OK TO CONTINUE PER MEL 49-15-01C. SP 1 AND #2. AUTH # 250402;[02] R/R APU AIR INLET DOOR ACTUATOR PER AMM 49-15-31. OPS CK GOOD & ALL SPS REVERSED.