As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER THE APU WAS STARTED AND THE BLEED AIR SELECTED ON THE CABIN IMMEDIATELY FILLED W/SMOKE. PERFORMED SPEED BRAKE OPERATIONAL CHECK IAW 400XP MM 27-60-00. OPS CHECKED GOOD. THE APU FAIL LIGHT CAME ON AND THE APU SHUT DOWN. REMOVED APU S/N P-403 AND INSTALLED SERVICEABLE APU UNIT S/N P-333 I.A.W CESSNA 560XL M/M CH 49-10-00.