The indoor unit of a split air conditioner houses the evaporator coil, blower fan, filter, and drain pan, each of which can develop issues independent of the outdoor unit's condition. A clogged filter is the most frequent cause of reduced airflow, restricting air across the evaporator coil and sometimes causing it to freeze over if left unaddressed for extended periods.
A frozen coil actually reduces cooling rather than improving it, since ice acts as an insulator and blocks proper heat exchange. The blower fan motor can wear out over years of continuous use, producing a rattling noise or reduced air throw even when the filter is clean.
Drain pans, positioned beneath the evaporator coil, collect condensate that forms as warm air passes over the cold coil; if this pan or its drain line becomes blocked with dust, algae, or biofilm, water can overflow into the room or drip down the wall beneath the unit. PCB boards inside the indoor unit control fan speed, timer functions, and communication with the outdoor unit, and issues here can cause erratic behavior like the unit not responding to remote commands or displaying error codes on models with digital displays.