A unit may be stuck in defrost mode for hree possible reasons. A mini split may cycle into defrost mode as a result of the following:
- Low on refrigerant or no pressure,
- Faulty outdoor circuit board,
- Defective outdoor discharge or coil sensor.
You can first set the unit on the maximum heating temperature, and verify if you are able to see the outdoor fan blade turn. If you see the outdoor blade turning and you can hear the compressor ramping up, then the circuit board should be fine.
Second test, after letting the unit run for approximately 10 to 15 min, you can feel the low side pressure valve, which is the larger one going to the air handler. If that valve is not hot at all, your system must have a restriction in the pressure or simply low on R410a refrigerant. You will then need a professional to verify the low side pressure or the refrigerant level, referring to the gray sticker on the side of your outdoor unit.
Third option, if the outdoor unit is running, and the lines are getting very hot but the indoor unit doesn't starts heating, it is possible for your indoor coil sensor to be faulty. If the sensor does not read the correct coil temperature, it will not allow the unit to complete the defrost and release the heat.