One unit showing error, others fine: indoor board dead

E1 showed on one bedroom unit while the other two in the system ran fine. A whole-system fault would hit all three, so the problem sat in that one unit's wiring or its indoor board. The job was to prove which, before quoting any part.

By Team Snowflake | Updated 28 Jul 2026

Case summary

Samsung Wall-mounted7-12 years oldCondoSengkang, Singapore

Concern: Either the outdoor unit or the whole system wiring had failed.
Found: Indoor unit control board communication circuit failure
Key check: control board swap test. E1 followed the board to the second room, confirming indoor control board fault
Result: We installed the replacement bedroom control board and E1 cleared straight away. All three units now run normally. The swap test took the guesswork out of the diagnosis, so the owner paid for one board and nothing more.

What we were told

One of three aircon units had shown an error code for about a week while the other two worked fine. The owner wanted to know whether it was a wiring problem or a fault inside the unit, and how much it would cost to fix.

What we checked

One unit faulting while the other two ran normally pointed to a fault inside that single indoor unit. The shared outdoor unit and the system wiring would affect all three if they were the cause, so we checked the local wiring first, then tested the board.

E1 stayed on the bedroom unit the whole time, and the unit would not start.
The other two indoor units ran normally with no errors, which ruled out a shared system fault.
The communication cable between the bedroom unit and the outdoor unit tested intact, with correct continuity end to end.
Wire connections at both ends were clean, with no corrosion to break the signal.
With wiring cleared, we swapped the bedroom control board into the living room unit. E1 followed the board to the living room, and the bedroom unit then ran fine on the living room board.

What we found

The communication circuit on the bedroom indoor control board had failed. That circuit carries the signal between the indoor board and the outdoor unit. Once it fails, the unit can no longer talk to the outdoor unit, and the system throws E1. The swap test settled it: the fault travelled with the board to the living room, while the bedroom unit ran fine on a working board. The wiring was sound, so the board was the only part left to blame.

What fixed it

We recommended replacing only the bedroom unit control board. The wiring, the outdoor unit, and the other two indoor units were all confirmed working, so there was nothing else to fix. We sourced the correct board for the model. At seven-plus years, the other boards may fail in time, but we do not replace parts that still work normally. The owner can address them if and when they fault.

Outcome

We installed the replacement bedroom control board and E1 cleared straight away. All three units now run normally. The swap test took the guesswork out of the diagnosis, so the owner paid for one board and nothing more.

What this case teaches us

One unit faulting in a multi-split points to that unit

When one unit errors and the others run fine, the fault is local to that unit, not the shared outdoor unit or system wiring.
A swap test isolates a board fault cheaply. If the error follows the board to another room, the board is the problem, not the wiring.
Confirm the part before ordering it. A E1 board swap takes a few minutes and rules out a wasted parts cost.

Questions this case answers

One aircon showing an error while the others work fine: where is the fault?

In that unit. A shared outdoor unit or shared system wiring would take all of them down, not one. On this Sengkang condo the other two indoor units ran normally throughout while the bedroom unit sat on E1 and would not start, which put the search inside that room from the beginning.

What is a control board swap test?

Moving the suspect board into a working unit and watching where the error goes. Here the bedroom board went into the living room unit, the code followed it, and the bedroom unit then ran fine on the living room board. That settles a board fault without guessing and without ordering a part first.

Do the other indoor boards need changing at the same time?

No. On a system of this age the remaining boards may fault eventually, but we do not replace components that are working normally. Only the bedroom board was changed. The other two can be dealt with if and when they actually fail, rather than pre-emptively on the strength of age alone.

How can a board fault be confirmed before the part is ordered?

Clear the wiring first, then swap. The cable between the faulting unit and the outdoor unit tested intact with correct continuity, and connections at both ends were clean with no corrosion. Only then does a swap mean anything. Ask for that sequence, because it is what keeps you from buying a part you do not need.

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