To understand a musty smell when the air conditioning starts, compare it under normal, safe conditions: cold versus warm, idle versus steady speed, and light versus higher load. Note whether the odor fades and whether it comes from one vent or all vents. Change one condition at a time so the result stays useful.
Do not create a risky test just to reproduce it. Check the cabin filter and follow the maker’s cleaning procedure; do not spray unapproved chemicals into the intake.
Quick educational note
For information, not a diagnosis or repair instruction. Vehicle-specific checks are still required. Consult the manufacturer's guidance and a qualified technician.
How to use this information
This article explains general principles and does not establish what is wrong with your vehicle. Read the content limitations.