A chemical overhaul is recommended when an aircon unit has been severely neglected for an extended period, typically more than a year without any cleaning, and has accumulated heavy mold, thick grease layers on the blower fan, or a severely clogged evaporator coil that cannot be adequately cleaned while the unit remains mounted. It is also recommended when the unit emits a strong persistent odor that does not go away after a chemical clean, when water is leaking consistently from the indoor unit due to a completely blocked drainage system, or when cooling performance remains poor even after a standard chemical clean has been performed. For older units being prepared for resale or reuse after a period of storage, a chemical overhaul is the most reliable way to restore them to a clean and functional condition.