What was found
DURING THE BOARDING PROCESS, THE CREW NOTIFIED MAINTENANCE OF A BURNING SMELL IN THE CABIN. THE APU WAS RUNNING AND THE PACKS WERE BEING OPERATED TO WARM THE CABIN. MAINTENANCE FOUND THE APU INLET CONTAMINATED WITH DE-ICE FLUID. THE APU INLET AND DOOR ASSEMBLY WERE CLEANED. THE PNEUMATIC SMOKE/ODOR IN THE CABIN TROUBLESHOOTING WORK CARD WAS ACCOMPLISHED. A DUCT BURNOUT WAS PERFORMED AND THE COALESCER BAGS WERE REPLACED. THE AIRCRAFT WAS RETURNED TO SERVICE.