As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SMOKE IN CABIN DURING TAXI OUT. TAXIED BACK TO THE GATE. GROUND RAN ALL, CHECKED LEFT TO RIGHT ENGINE BLEEDS AT DIFFERENT POWER SETTINGS. NO SMOKE NOTED DURING ENGINE RUN. WHEN APU WAS SHUT DOWN THE CABIN AND COCKPIT FILLED UP WITH SMOKE. SUSPECT APU COMPRESSOR SEAL LEAKING. REMOVED AND REPLACED APU WITH REPAIRED UNIT, PER MM 49-00-02.