All four rooms down at once: outdoor board died

Four indoor units in one flat showed CH05 at the same moment. When every unit in a multi-split fails together, the cause is usually the one part they all share. A full replacement looked likely at first, but the fault turned out to be smaller and on the outdoor unit.

By Team Snowflake | Updated 28 Jul 2026

Case summary

LG Multi-split7-12 years oldCondoTampines, Singapore

Concern: The whole multi-split system looked to have failed and to need replacing.
Found: Outdoor unit control board communication circuit failure
Key check: Signal check at the connection showed no output. Outdoor board not transmitting to any indoor unit
Result: We replaced the outdoor control board, and all four indoor units returned to normal cooling. CH05 cleared across the whole system, and there have been no further faults since the repair.

What we were told

All four units stopped working at the same time, and each one showed the same error code. Nothing had changed. There was no power trip and no renovation. It just happened one morning.

What we checked

All four indoor units showing the same fault at once pointed to one shared cause, not four separate ones. In a multi-split, the outdoor unit is the part every indoor unit relies on, so we started there.

CH05 showed on all four indoor units, and none would start.
CH05 is a communication error, so we tested the cable that carries the signal between each indoor unit and the outdoor unit. Every line was intact.
The wire connections at both the indoor and outdoor ends were clean, with no corrosion or loose terminals.
With the wiring ruled out, we checked for the signal itself at the outdoor connection. There was no output, so the board was not sending anything.
The indoor boards were getting power normally but had no signal to respond to, which is why all four reported the same error.

What we found

The communication circuit on the outdoor control board had failed. The board was no longer producing the signal that keeps the indoor units in step with the outdoor unit. All four indoor units depend on this one board for that signal, so all four showed CH05 together. The fault was limited to the communication circuit. The power supply and compressor control parts of the board still worked, which is why the rest of the system was fine.

What fixed it

We recommended replacing the outdoor control board. The failure was isolated to that one board, and the rest of the system was in working condition. The compressor, fan motor, and all four indoor units tested fine. We sourced the exact replacement board for this system model rather than a generic part. We told the homeowner that on a system this age other outdoor components may need attention later, but there was no reason to replace anything that was still working now.

Outcome

We replaced the outdoor control board, and all four indoor units returned to normal cooling. CH05 cleared across the whole system, and there have been no further faults since the repair.

What this case teaches us

When every unit fails at once, look at the part they share

A multi-split runs its indoor units off one outdoor unit. When all of them fail together, the shared outdoor unit is the first place to check.
The same error on every unit does not mean every unit is broken. Here only one circuit on the outdoor board had failed, and the rest of the system was sound.
A signal test at the outdoor connection separates a wiring fault from a board fault. That single check decided whether to replace a board or the whole system.

Questions this case answers

All four aircon units showing the same error at once: is the whole system finished?

Usually not. A multi-split runs every indoor unit off one outdoor unit, so a single shared part can take all of them down together. In this Tampines flat only the communication circuit on the outdoor board had failed. The compressor, the fan motor and all four indoor units tested sound and were kept.

Can one failed circuit stop every room in a multi-split?

Yes, when the failed circuit is the one they all depend on. The outdoor board sends the signal that keeps the indoor units in step. Once it stopped producing that signal, all four reported the same fault at the same moment. Power supply and compressor control on the board still worked normally.

What separates a wiring fault from a board fault on a multi-split?

A signal test at the outdoor connection point. If wiring is to blame, the cable or the terminals show it. Every line was intact here and every terminal was clean, so we looked for the signal itself and found no output at all. That one check decided between a board and a whole system.

Should the indoor units be replaced at the same time as the outdoor board?

Not if they still work. Every indoor board in this flat was receiving power normally and simply had nothing to respond to. We source the exact matching board for the system model rather than a generic part. On an older system other outdoor components may need attention later, but nothing sound gets swapped now.

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