Why did all four units show the same error at once?

In a multi-split system, all indoor units communicate with a single outdoor unit. When the outdoor PCB stops transmitting, every connected indoor unit loses its communication link and displays the same error code.

Can the outdoor PCB be repaired instead of replaced?

In some cases, a specific failed component on the board can be identified and replaced. However, for communication circuit failures, board-level repair is often impractical because the fault may involve multiple micro-components. A full board replacement is more reliable.

How long does an outdoor PCB replacement take?

The replacement itself takes one to two hours once the part is available. Lead time for sourcing the correct board varies — common LG models are typically available within a few days.

LG CH05 Outdoor PCB Communication Failure on System-4

Aircon case in Tampines, Singapore: electrical/control traced to outdoor unit PCB communication circuit failure after targeted diagnosis checks.

Case Details

Reported: All four units stopped working at the same time. They are all showing the same error code. Nothing changed — no power trip, no renovation. It just happened one morning.
Unit: LG · Multi-split · 7–12 years
Location: Condo · Tampines, Singapore

What We Checked

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 Diagnosis

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.

What Fixed It

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. No further faults have appeared since the repair.

Why This Happens

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 is significantly less costly than replacing the entire system, but the board must match the exact model to maintain compatibility with all connected indoor units.

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