All Four Rooms Down At Once: Outdoor Board Died

• •Concern: Worry was that the entire multi-split system needed replacing
• •Key check: Signal voltage on S-terminal measured flat. Outdoor board not transmitting to any indoor unit

All four indoor units showing the same fault simultaneously pointed to a shared upstream cause. We focused on the outdoor unit as the common link.

• •CH05 displayed on all four indoor units. None would start.
• •Communication cable continuity tested intact between each indoor unit and the outdoor unit.
• •Terminal connections at both indoor and outdoor ends showed no corrosion or looseness.
• •Signal voltage measurement on the outdoor unit S-terminal showed no output, the board was not transmitting.
• •Indoor unit boards were receiving power normally but had no communication signal to respond to.

The outdoor unit PCB communication circuit had failed. The board was no longer generating the signal that coordinates with the indoor units. Because all four indoor units depend on this single outdoor board for communication, all four showed CH05 at the same time. The failure was confined to the communication circuit, the power supply and compressor control sections of the board were still functional.

We recommended replacing the outdoor PCB. The communication circuit failure was isolated to the board, and the rest of the system. Compressor, fan motor, indoor units. Was in working condition. We sourced the exact replacement board for the system model. We also advised the homeowner that on a system this age, other outdoor components may eventually need attention, but there was no reason to preemptively replace anything that was still functioning.

The outdoor PCB was replaced and all four indoor units resumed normal operation. CH05 cleared across the entire system with no further faults since the repair.

How simultaneous errors across all units narrow the fault to one point.

• •In a multi-split system, the outdoor unit is the single shared component. When all indoor units fault simultaneously, the outdoor unit is almost always the source.
• •A flat signal voltage on the communication terminal confirms the outdoor PCB is not transmitting. This rules out wiring and indoor boards in one measurement.
• •Replacing an outdoor PCB costs far less than replacing the entire system. The board must match the exact model, though, to stay compatible with all connected indoor units.