Ductless mini-split heat pump / AC: field service manual

Inverter concept: the outdoor board rectifies incoming AC to a high DC bus, then synthesizes variable-frequency power for the compressor (and often a DC fan motor). The compressor ramps up on a big load and slows down near setpoint rather than stopping.

Communication: indoor and outdoor boards exchange data over the interconnect cable. On the common 3-wire layout (often S1/S2/S3 or 1/2/3), two wires carry power and the third carries a data signal referenced to one of them. A healthy data line typically reads as a fluctuating DC voltage on a meter; a steady or missing signal points to wiring or a board. Exact values differ by brand: use the service manual.

Sensors: each indoor head has at least a room/return-air thermistor and a coil thermistor; the outdoor unit adds ambient, coil, discharge and sometimes suction sensors. Midea-platform brands (MrCool, Pioneer, some Carrier) label them T1 room, T2 indoor coil, T3 outdoor coil, T4 outdoor ambient, T5 discharge. Mitsubishi calls them TH1, TH2 and so on.

Error display: faults appear as a code on the wired controller or display, or as blink counts on the Operation/Timer LEDs. Count and record the blinks before resetting power, because a breaker reset clears the active code.

Condensate: wall heads drain by gravity through a line that must fall continuously; cassettes and ducted units, and wall heads that cannot drain downhill, use a condensate pump with a float that stops the system if water backs up.

Heat pump models reverse the refrigerant flow for heating and defrost the outdoor coil periodically; during defrost the indoor fan stops or runs very slowly and the outdoor unit may steam. That is normal.