The indoor and the outdoor unit exchange signals over the connecting wire all the time. When one side does not hear the other for a while, the unit stops and reports a communication error.
Common causes
- The wire between the units is connected in the wrong order, loose, broken or chewed; a joint along the run is damp.
- The outdoor unit's supply is unstable or missing.
- Electrical noise from other equipment on the same line.
- A faulty communication circuit on the indoor or the outdoor board.
What to check, in order
- Compare with the wiring diagram on the cover: every wire on the right terminal, in the right order, at both ends.
- Tighten the terminals; look for joints along the run, burn marks, crushed cable.
- Power off, disconnect both ends, check continuity and insulation of each wire.
- Power on, measure the outdoor supply and the signal line as the manual of the exact model describes (the reading usually swings while the signal is being sent).
- Look at the lamps on the outdoor board: if the board does not run, look for the cause in its supply and fuse before blaming the communication circuit.
- Replace a board only when all of the above pass.
The voltages, the right way to measure and each model's own procedure are in the service manual: open the matching code below in the app.
Safety: switch off both the indoor and the outdoor unit before opening a cover or touching a board; the capacitors on the outdoor board stay charged for a while after power-off - wait for the board's lamps to go out and measure before touching. Refrigerant and high-voltage work belongs to a certified technician.