As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT RETURNED TO GATE ON ACCOUNT OF CABIN FILLED WITH SMOKE FROM AIR CONDITIONING SYSTEM. APU REMOVED AND OPERATED IN TEST CELL, NO EVIDENCE OF OIL LOSS OR PROBABILITY OF DUCT CONTAMINATION. OIL LOSS COULD HAVE BEEN OVERSERVICING. APU RECEIVED C-CHECK AND WAS RETURNED TO SERVICE.