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Error Code
UA means the system has decided the indoor and outdoor units are not a matching pair, or can no longer coordinate as one. Daikin equipment is built to be paired by model, and the board checks that both halves agree on capacity and control protocol. When they do not, it refuses to run. That is often the wrong unit fitted, and sometimes a wiring or board fault that mimics one.
A replacement head or condenser fitted from a different series will power up and then report UA, because the two boards do not speak the same protocol or agree on capacity. It happens after a partial replacement, or when a unit has been sourced secondhand. The model numbers on both halves settle it in a minute.
Wrong interconnecting wiring between the two units, a multi core cable landed on the wrong terminals, or a two core run where the system expects three will all produce UA. It is a commissioning fault rather than a wear fault, so it shows on a new install or straight after a repair. The wiring diagram is the reference.
A damaged conductor between the units, or an indoor or outdoor board that has failed on its communication port, gives the same symptom as an incompatible pair. Continuity and voltage on the comms pair separate a wiring fault from a board fault, and that distinction matters because one is cheap and the other is not.
The model and serial numbers on both halves are checked against the manufacturer pairing tables before anything electrical is touched. A mismatch ends the diagnosis. It is the first check on UA because it is free, it cannot be fixed by repair, and it explains the code completely when it is the cause.
The interconnecting cable is traced end to end, continuity tested on each core, and the terminals at both boards checked for tightness and corrosion. Correct polarity and the right number of cores both matter here. Where the cable is sound, the boards are tested on the comms port itself.
The installation section lists the wiring arrangement, the cable specification and the commissioning steps for that specific pair. Working from the manual matters on UA because the required connection differs between single phase, three phase and multi head systems, and a generic guess will not do.
