Capacitor swap didn't fix it: outdoor board was the fault

A capacitor swap two weeks earlier had not held. The unit ran for three days, then threw the same error code and stopped. A repeat fault after a part swap usually means the cause was never found, not that a second part has failed.

By Team Snowflake | Updated 28 Jul 2026

Case summary

Mitsubishi Heavy Industries Wall-mounted8 years oldHDBJurong East, Singapore

Concern: The owner faced a full outdoor unit replacement quote after the capacitor swap had not fixed the fault.
Found: Outdoor control board output fault. No output signal to compressor
Key check: Checked power at the control-board start output to the compressor wiring. The signal was absent despite correct input power, confirming a control-board fault
Result: The unit has run without fault since the board was replaced. The owner avoided a full outdoor unit swap and the piping work that would have come with it.

What we were told

The run capacitor had been replaced two weeks earlier. The unit worked for three days, then threw the same error code and stopped. A full outdoor unit replacement was the next step on the table. The owner wanted a second opinion before spending on a new unit.

What we checked

The capacitor had already been swapped, so we first checked whether that new part was actually healthy. It was. That ruled out the obvious suspect. We then traced the outdoor circuit in order, looking for the point where the start command stopped before reaching the compressor.

The recently fitted capacitor tested normal, so it was not the cause of the repeat fault.
The starter relay and its contacts tested normal, with no burn marks, and the coil pulled in cleanly when powered.
Power into the outdoor wiring point measured normal, so the supply was not the problem.
The control board received power but was not sending the start signal out to the compressor.
Compressor checks came back normal, so the motor was healthy and waiting for a signal that never arrived.

What we found

The outdoor control board had failed at the section that sends the start signal to the compressor. Power reached the board and the capacitor was fine, but the board did not pass the start command through. The board had been failing on and off for a while. The brief recovery after the capacitor swap was a coincidence, not a fix. Eventually the board stopped sending the signal at all. The compressor was undamaged. It simply never got the instruction to start.

What fixed it

We sourced a matching control board for the MHI model and fitted it to the outdoor unit, carrying over the configuration settings from the original board. On power-up, the compressor received the start signal normally. A full cooling cycle confirmed the indoor unit reached temperature with no error returning. The compressor, starter relay, and recently fitted capacitor were all healthy, so we kept them. Only the control board was replaced.

Outcome

The unit has run without fault since the board was replaced. The owner avoided a full outdoor unit swap and the piping work that would have come with it.

What this case teaches us

A repeat fault after a part swap means the cause was missed

The capacitor was swapped, the unit recovered for three days, then failed again. A short recovery does not prove the right part was changed.
The board sends the start signal to the compressor. When that signal is missing, the compressor sits idle even though it is healthy.
Before agreeing to replace the whole outdoor unit, ask whether the control board output to the compressor was actually tested.

Questions this case answers

Aircon works for a few days after a repair then fails again: what does that mean?

A short recovery does not prove the right part was changed. On this Jurong East unit the same error code returned after a few good days, so the checks started again from the beginning. The capacitor tested normal, the starter relay and its contacts were sound, and incoming power measured correct. The fault sat further along, at the outdoor control board.

Compressor tests fine but never starts: what else stops it?

Yes. The board sends the start command, and if that output fails, the compressor sits idle waiting for an instruction that never arrives. Compressor checks here came back normal, and power reached the board without the start signal leaving it. From outside the unit looks dead either way, which is why that output has to be measured.

The same error code keeps returning: is a bigger part failing?

Not by itself. A returning code more often means the cause was never located, and this board had been failing on and off before it stopped passing the signal at all. Ask whether the control board output to the compressor was tested. That check separates a board replacement from a full outdoor unit replacement.

Can just the outdoor control board be replaced?

Yes, when everything around it tests healthy. A matching board for this MHI model was fitted with the original configuration settings carried across, and the compressor took its start signal normally on power-up. The compressor, starter relay and capacitor were all sound and stayed in place, so a full outdoor swap and its piping work were avoided.

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