Why did replacing the capacitor not fix it?

The capacitor was not the fault here — the PCB driver circuit had failed. Both faults stop the outdoor unit from starting, which is why the capacitor gets replaced first. The swap was a reasonable first step; it just did not fix the real cause. Without probing the PCB output signal, there is no way to tell these two faults apart by symptoms alone.

How do you know it's the PCB and not the compressor?

We measured voltage at the PCB output terminals to the compressor during a start command. No signal was present. That means the compressor never got the instruction to start. We did not test the compressor under load because the start signal was never delivered. If the PCB had been working and the compressor still failed to start, we would have moved to a winding test next.

Is an 8-year-old unit worth fixing the PCB on?

Eight years is mid-life for a well-kept unit. If the compressor, refrigerant circuit, and other major parts are in good shape — as they were here — a PCB replacement is a straightforward fix that extends the life of the system. We would only avoid a PCB repair if the unit had several faults at once or if compressor wear was already showing.

Outdoor unit error traced to PCB fault after capacitor ruled out

Aircon case in Jurong East, Singapore: electrical/control traced to outdoor PCB driver circuit fault — no output signal to compressor after targeted diagnosis checks.

Case details

What client reported

The customer said a previous technician had replaced the run capacitor two weeks earlier. The unit ran for three days, then threw the same error code and stopped. They wanted a second opinion before the previous tech replaced the whole outdoor unit.

ProblemElectrical / control

UnitMitsubishi Heavy Industries · Wall-mounted · 8 years old

LocationHDB · Jurong East, Singapore

What we found

We checked the earlier repair first before testing deeper in the outdoor unit circuit

New capacitor measured — within tolerance, not the cause of the recurring fault
Contactor tested — coil and contacts both normal, no arcing damage
Mains voltage at outdoor terminal block confirmed normal — supply was not the issue
PCB driver output to compressor — no signal during a start command, board confirmed faulty

The outdoor PCB driver circuit had failed. The board had correct mains power and the capacitor was fine. But the driver stage had no output to the compressor. This is why the capacitor swap seemed to work briefly — the board had recovered before failing for good.

What we did

The outdoor PCB was the faulty part, not the whole outdoor unit. We sourced a board for the MHI model and fitted it. We then checked that the start signal reached the compressor. The unit ran through a full cooling cycle with no error code.

The unit has run without fault since the board was replaced. The customer avoided the cost of a full outdoor unit replacement. The previous contractor had proposed this without running the voltage trace that would have found the PCB fault.

Timeline

Day 1

Error code and non-starting outdoor unit — previous capacitor replacement had not resolved the fault

Day 2

Voltage measured at PCB driver output to compressor terminals showed signal absent despite correct input power — PCB confirmed faulty

Day 2

Capacitor re-verified, contactor tested, PCB driver output traced — board confirmed faulty, replaced, unit running

What we learned

Why replacing the capacitor doesn't always fix a non-starting outdoor unit.

Capacitor failure and PCB driver failure both stop the outdoor unit from starting. The capacitor is replaced first because it is cheap and the most common cause in units under 10 years.
Error codes narrow the search but do not name one component. The same code can come from a failed capacitor, a faulty PCB, or a compressor fault. Each needs a different test.
If the unit still will not start after a new capacitor is fitted, the PCB driver circuit is the next likely cause. It sends the start signal to the compressor.
Measuring voltage at the PCB output to the compressor is what separates these two faults. If the signal is absent despite correct mains input, the board has failed.

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