Daikin Mini-Split Not Cooling But Running: Refrigerant, Filter, and U4 Diagnosis

A Daikin mini-split that runs (indoor fan blowing air) but doesn't cool indicates the outdoor unit is not producing cold refrigerant. Check the outdoor unit: if the fan spins but the compressor is not humming, the unit has a fault preventing compressor operation. The most common cause is the U4 communication fault between indoor and outdoor units - check the S1-S2 wiring terminals. If the compressor IS running but air is not cold: low refrigerant from a leak. Feel the large copper line at the outdoor unit - it should be cold and sweating in cooling mode. If it is warm or room temperature, refrigerant is low. Daikin inverter compressors will continue running at reduced capacity even with low charge (unlike non-inverter systems that simply shut off), so you may get slightly cool but not cold air. Clean the indoor filters (behind the front panel, lift and slide out) and the outdoor condenser coil monthly.