Aircon PCB Fault, Not Capacitor

Case Details

What We Checked

• Recently fitted capacitor measured within tolerance at 14.8 µF against a 15 µF rating — not the cause of the recurring fault.
• Contactor coil and contacts both tested normal with no arcing damage. Coil pulled in cleanly when energised.
• Mains voltage at outdoor terminal block confirmed within range at 231V.
• PCB driver output showed no signal to the compressor during a start command — zero volts where the board should have been delivering a controlled output.
• Compressor winding resistance measured normal across all three terminals, confirming the motor was healthy and waiting for a signal that never arrived.

The Diagnosis

The outdoor PCB driver circuit had failed. Specifically, the power transistor stage responsible for converting mains input into a controlled start signal for the compressor had degraded. Mains power reached the board and the capacitor was fine, but the driver output measured zero volts at the compressor terminals during a start command. The brief recovery after the capacitor swap was coincidental — the driver stage had been intermittently failing for weeks, producing a weakening signal that eventually dropped below the threshold needed to energise the compressor. Once it failed completely, no amount of capacitor work could restore the missing output. The compressor itself was undamaged — it simply never received the instruction to start.

What Fixed It

We sourced a matching replacement PCB for the MHI model and fitted it to the outdoor unit, transferring the configuration settings from the original board. After powering up, we measured the driver output at the compressor terminals during a start command — the signal was now present and within specification. We ran a full cooling cycle, monitoring supply current at steady state and compressor discharge pressure throughout to confirm the compressor was operating within its rated envelope. The indoor unit reached the set temperature within the expected time and no error codes appeared on either board. The compressor, contactor, and recently replaced capacitor were all confirmed healthy and retained — only the PCB was replaced.

The unit has run without fault since the board replacement. No outdoor unit swap or piping work was needed.

Why This Happens

What to check when a capacitor swap does not fix the fault.

• The capacitor is the right first suspect for a non-starting outdoor unit — it is cheap, common, and easy to test. But if a new capacitor does not resolve the error within a few days, the diagnosis needs to move deeper into the circuit. A brief recovery after a swap does not confirm the capacitor was the root cause; it may simply mean the intermittent fault happened to pause temporarily.
• The PCB driver stage converts mains power into a controlled signal for the compressor motor. Measuring voltage at the driver output terminals during a start command confirms whether the board is delivering that signal. Zero volts at a point that should show a controlled output is definitive. This measurement takes minutes and clearly separates a board fault from a compressor fault.
• Error codes point to a symptom area, not a single component. The same error code can come from a degraded capacitor, a failed driver circuit, a compressor winding fault, or even a loose terminal connection. Each requires a different measurement to confirm. Ask your technician which specific test — capacitance reading, driver output voltage, or winding resistance — confirmed their diagnosis.
• An intermittently failing PCB can mimic capacitor problems because both cause the outdoor unit to stop starting with the same error code. The key difference is measurable: a capacitor fault shows a significant drop in capacitance on a meter, while a PCB driver fault shows missing or unstable output voltage at the compressor terminals. If the capacitor reads within tolerance, the board is the next suspect in the diagnostic sequence.