New Install Acting Strange: Two Wires Swapped at Install

One room's remote doing nothing to its own unit and starting the neighbouring room instead, every time without exception. Both rooms otherwise run and cool normally once something has started them. The behaviour has been there since the system went in.

Repeatability is the discriminator, and it is free to observe. Electronics fail intermittently and in ways that shift; a defective board does not politely activate the same wrong room on every attempt. A response that is wrong in a fixed, symmetrical way describes two things connected to each other's places. That is a terminal block, not a component.

Each indoor unit on a multi-split has its own signal pair, carrying on and off, setpoint and error codes for that zone alone. Cross two pairs at the outdoor terminals and each remote reaches the other unit, exactly and permanently. Nothing is damaged and nothing is marginal, which is why the symptom never varies and never worsens. The equipment is doing precisely what it has been told to do, by the terminals it was told through.

Move the two pairs back to their proper terminals, then switch on each room singly and watch only that room answer. What is worth the extra five minutes is labelling both ends of every pair, because the next visit inherits whatever the last one left and an unlabelled terminal block invites the same mistake twice. No parts, and no warranty claim.