Evaporator coil

The evaporator coil sits inside the indoor unit, where cold refrigerant absorbs heat from the room air blown across it — the part that actually makes the air cold and pulls humidity out of it. When it ices over or clogs with dust and biofilm you get weak, warm airflow and often a damp smell. Clean filters are what protect this coil, which is why a simple filter clean is the cheapest thing you can do for cooling performance.