Samsung AQV12VBCN Vivace Mirror Finish Wall Mounted Mini Split Heat Pump - 12 "heated mirrors turned" Questions

What causes black, soot-like dirt in heating ducts? Getting a burnt smell when I turn on the furnace. Blower has been cleaned, hall air-filter is clean. I used a long vacuum hose to get down the vents

mini split Samsung aqv09ja flashing but no turn on Ive seen and heard of this fault before. Most of the time its a failed outdoor Electronic board but sometimes, it can be the indoor as well. Be safe ...