Capacitor swap didn't fix it: outdoor board was the actual fault
Another contractor had already replaced the capacitor two weeks earlier. 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 | Reviewed 3 Mar 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 control board start output to compressor wire connections showed signal absent despite correct input power. control board confirmed faulty
- 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
Another contractor had replaced the run capacitor two weeks ago. The unit worked for three days, then threw the same error code and stopped. That contractor then quoted a full outdoor unit replacement. 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. We ran a full cooling cycle and 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.
Related reading
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