What was found
DURING PREFLIGHT, WITH APU BLEED RUNNING, CABIN FILLED WITH BLUE SMOKE. FIRST OFFICER INDICATED CABIN WAS VERY HOT EVEN THOUGH TEMP SETTING WAS VERY LOW. REMOVED DEFECTIVE APU & REPLACED WITH REPAIRED APU. CLEANED APU CONTAINMENT BOX & FLUSHED DRAIN LINES. CHANGED CONDENSER SOCK & PERFORMED DUCT BURNOUT. NO FURTHER SMOKE OR ODORS IN CABIN OR COCKPIT. SERVICED, OPERATIONAL & LEAK CHECKS WERE FOUND TO BE GOOD AT THIS TIME. ALL WORK COMPLETED IAW MM 49-00-01. CONTAINMENT BOX ACCESS & 49-10-00, FOR APU R & R, BUILDUP, & OPERATIONAL CHECKS.